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cmd/gc: handle non-escaping address-taken variables better

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This CL makes the bitmaps a little more precise about variables
that have their address taken but for which the address does not
escape to the heap, so that the variables are kept in the stack frame
rather than allocated on the heap.

The code before this CL handled these variables by treating every
return statement as using every such variable and depending on
liveness analysis to essentially treat the variable as live during the
entire function. That approach has false positives and (worse) false
negatives. That is, it's both sloppy and buggy:

        func f(b1, b2 bool) {	// x live here! (sloppy)
                if b2 {
                        print(0) // x live here! (sloppy)
                        return
                }
                var z **int
                x := new(int)
                *x = 42
                z = &x
                print(**z) // x live here (conservative)
                if b2 {
                        print(1) // x live here (conservative)
                        return
                }
                for {
                        print(**z) // x not live here (buggy)
                }
        }

The first two liveness annotations (marked sloppy) are clearly
wrong: x cannot be live if it has not yet been declared.

The last liveness annotation (marked buggy) is also wrong:
x is live here as *z, but because there is no return statement
reachable from this point in the code, the analysis treats x as dead.

This CL changes the liveness calculation to mark such variables
live exactly at points in the code reachable from the variable
declaration. This keeps the conservative decisions but fixes
the sloppy and buggy ones.

The CL also detects ambiguously live variables, those that are
being marked live but may not actually have been initialized,
such as in this example:

        func f(b1 bool) {
                var z **int
                if b1 {
                        x := new(int)
                        *x = 42
                        z = &x
                } else {
                        y := new(int)
                        *y = 54
                        z = &y
                }
                print(**z) // x, y live here (conservative)
        }

Since the print statement is reachable from the declaration of x,
x must conservatively be marked live. The same goes for y.
Although both x and y are marked live at the print statement,
clearly only one of them has been initialized. They are both
"ambiguously live".

These ambiguously live variables cause problems for garbage
collection: the collector cannot ignore them but also cannot
depend on them to be initialized to valid pointer values.

Ambiguously live variables do not come up too often in real code,
but recent changes to the way map and interface runtime functions
are invoked has created a large number of ambiguously live
compiler-generated temporary variables. The next CL will adjust
the analysis to understand these temporaries better, to make
ambiguously live variables fairly rare.

Once ambiguously live variables are rare enough, another CL will
introduce code at the beginning of a function to zero those
slots on the stack. At that point the garbage collector and the
stack copying routines will be able to depend on the guarantee that
if a slot is marked as live in a liveness bitmap, it is initialized.

R=khr
CC=golang-codereviews, iant
https://golang.org/cl/51810043
This commit is contained in:
Russ Cox 2014-01-16 10:32:30 -05:00
parent fbfb9430dc
commit ca9975a45e
5 changed files with 541 additions and 109 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

12
src/cmd/gc/bv.c
View File

@ -135,6 +135,18 @@ bvor(Bvec *dst, Bvec *src1, Bvec *src2)
dst->b[w] = src1->b[w] | src2->b[w]; dst->b[w] = src1->b[w] | src2->b[w];
} }
/* intersection */
void
bvand(Bvec *dst, Bvec *src1, Bvec *src2)
{
int32 i, w;
if(dst->n != src1->n || dst->n != src2->n)
fatal("bvor: lengths %d, %d, and %d are not equal", dst->n, src1->n, src2->n);
for(i = 0, w = 0; i < dst->n; i += WORDBITS, w++)
dst->b[w] = src1->b[w] & src2->b[w];
}
void void
bvprint(Bvec *bv) bvprint(Bvec *bv)
{ {

1
src/cmd/gc/go.h
View File

@ -1036,6 +1036,7 @@ int bvget(Bvec *bv, int32 i);
int bvisempty(Bvec *bv); int bvisempty(Bvec *bv);
void bvnot(Bvec *bv); void bvnot(Bvec *bv);
void bvor(Bvec *dst, Bvec *src1, Bvec *src2); void bvor(Bvec *dst, Bvec *src1, Bvec *src2);
void bvand(Bvec *dst, Bvec *src1, Bvec *src2);
void bvprint(Bvec *bv); void bvprint(Bvec *bv);
void bvreset(Bvec *bv, int32 i); void bvreset(Bvec *bv, int32 i);
void bvresetall(Bvec *bv); void bvresetall(Bvec *bv);

3
src/cmd/gc/pgen.c
View File

@ -293,7 +293,8 @@ cmpstackvar(Node *a, Node *b)
return +1; return +1;
if(a->type->width > b->type->width) if(a->type->width > b->type->width)
return -1; return -1;
return 0;
return strcmp(a->sym->name, b->sym->name);
} }
// TODO(lvd) find out where the PAUTO/OLITERAL nodes come from. // TODO(lvd) find out where the PAUTO/OLITERAL nodes come from.

553
src/cmd/gc/plive.c
View File

@ -2,6 +2,17 @@
// Use of this source code is governed by a BSD-style // Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file. // license that can be found in the LICENSE file.
// Garbage collector liveness bitmap generation.
// The command line flag -live causes this code to print debug information.
// The levels are:
//
// -live (aka -live=1): print liveness lists as code warnings at safe points
// -live=2: print an assembly listing with liveness annotations
// -live=3: print information during each computation phase (much chattier)
//
// Each level includes the earlier output as well.
#include <u.h> #include <u.h>
#include <libc.h> #include <libc.h>
#include "gg.h" #include "gg.h"
@ -73,29 +84,48 @@ struct Liveness {
Array *vars; Array *vars;
// A list of basic blocks that are overlayed on the instruction list. // A list of basic blocks that are overlayed on the instruction list.
// The blocks are roughly in the same order as the instructions
// in the function (first block has TEXT instruction, and so on).
Array *cfg; Array *cfg;
// Summary sets of block effects. The upward exposed variables and // Summary sets of block effects.
// variables killed sets are computed during the dataflow prologue. The // The Bvec** is indexed by bb->rpo to yield a single Bvec*.
// live in and live out are solved for and serialized in the epilogue. // That bit vector is indexed by variable number (same as lv->vars).
//
// Computed during livenessprologue using only the content of
// individual blocks:
//
// uevar: upward exposed variables (used before set in block)
// varkill: killed variables (set in block)
// avarinit: addrtaken variables set or used (proof of initialization)
//
// Computed during livenesssolve using control flow information:
//
// livein: variables live at block entry
// liveout: variables live at block exit
// avarinitany: addrtaken variables possibly initialized at block exit
// (initialized in block or at exit from any predecessor block)
// avarinitall: addrtaken variables certainly initialized at block exit
// (initialized in block or at exit from all predecessor blocks)
Bvec **uevar; Bvec **uevar;
Bvec **varkill; Bvec **varkill;
Bvec **livein; Bvec **livein;
Bvec **liveout; Bvec **liveout;
Bvec **avarinit;
Bvec **avarinitany;
Bvec **avarinitall;
// An array with a bit vector for each safe point tracking live pointers // An array with a bit vector for each safe point tracking live pointers
// in the arguments and locals area. // in the arguments and locals area, indexed by bb->rpo.
Array *argslivepointers; Array *argslivepointers;
Array *livepointers; Array *livepointers;
// An array with a bit vector for each safe point tracking dead values // An array with a bit vector for each safe point tracking dead values
// pointers in the arguments and locals area. // pointers in the arguments and locals area, indexed by bb->rpo.
Array *argsdeadvalues; Array *argsdeadvalues;
Array *deadvalues; Array *deadvalues;
}; };
static int printnoise = 0;
static void* static void*
xmalloc(uintptr size) xmalloc(uintptr size)
{ {
@ -600,10 +630,20 @@ isfunny(Node *node)
return 0; return 0;
} }
// Computes the upward exposure and kill effects of an instruction on a set of // Computes the effects of an instruction on a set of
// variables. The vars argument is an array of Node*s. // variables. The vars argument is an array of Node*s.
//
// The output vectors give bits for variables:
// uevar - used by this instruction
// varkill - set by this instruction
// avarinit - initialized or referred to by this instruction,
// only for variables with address taken but not escaping to heap
//
// The avarinit output serves as a signal that the data has been
// initialized, because any use of a variable must come after its
// initialization.
static void static void
progeffects(Prog *prog, Array *vars, Bvec *uevar, Bvec *varkill) progeffects(Prog *prog, Array *vars, Bvec *uevar, Bvec *varkill, Bvec *avarinit)
{ {
ProgInfo info; ProgInfo info;
Adr *from; Adr *from;
@ -614,6 +654,8 @@ progeffects(Prog *prog, Array *vars, Bvec *uevar, Bvec *varkill)
bvresetall(uevar); bvresetall(uevar);
bvresetall(varkill); bvresetall(varkill);
bvresetall(avarinit);
proginfo(&info, prog); proginfo(&info, prog);
if(prog->as == ARET) { if(prog->as == ARET) {
// Return instructions implicitly read all the arguments. For // Return instructions implicitly read all the arguments. For
@ -626,13 +668,6 @@ progeffects(Prog *prog, Array *vars, Bvec *uevar, Bvec *varkill)
case PPARAMOUT: case PPARAMOUT:
bvset(uevar, i); bvset(uevar, i);
break; break;
case PAUTO:
// Because the lifetime of stack variables
// that have their address taken is undecidable,
// we conservatively assume their lifetime
// extends to the return as well.
if(isfat(node->type) || node->addrtaken)
bvset(uevar, i);
} }
} }
return; return;
@ -659,6 +694,8 @@ progeffects(Prog *prog, Array *vars, Bvec *uevar, Bvec *varkill)
pos = arrayindexof(vars, from->node); pos = arrayindexof(vars, from->node);
if(pos == -1) if(pos == -1)
goto Next; goto Next;
if(from->node->addrtaken)
bvset(avarinit, pos);
if(info.flags & (LeftRead | LeftAddr)) if(info.flags & (LeftRead | LeftAddr))
bvset(uevar, pos); bvset(uevar, pos);
if(info.flags & LeftWrite) if(info.flags & LeftWrite)
@ -678,6 +715,8 @@ Next:
pos = arrayindexof(vars, to->node); pos = arrayindexof(vars, to->node);
if(pos == -1) if(pos == -1)
goto Next1; goto Next1;
if(to->node->addrtaken)
bvset(avarinit, pos);
if(info.flags & (RightRead | RightAddr)) if(info.flags & (RightRead | RightAddr))
bvset(uevar, pos); bvset(uevar, pos);
if(info.flags & RightWrite) if(info.flags & RightWrite)
@ -711,6 +750,9 @@ newliveness(Node *fn, Prog *ptxt, Array *cfg, Array *vars, int computedead)
result->varkill = xmalloc(sizeof(Bvec*) * nblocks); result->varkill = xmalloc(sizeof(Bvec*) * nblocks);
result->livein = xmalloc(sizeof(Bvec*) * nblocks); result->livein = xmalloc(sizeof(Bvec*) * nblocks);
result->liveout = xmalloc(sizeof(Bvec*) * nblocks); result->liveout = xmalloc(sizeof(Bvec*) * nblocks);
result->avarinit = xmalloc(sizeof(Bvec*) * nblocks);
result->avarinitany = xmalloc(sizeof(Bvec*) * nblocks);
result->avarinitall = xmalloc(sizeof(Bvec*) * nblocks);
nvars = arraylength(vars); nvars = arraylength(vars);
for(i = 0; i < nblocks; i++) { for(i = 0; i < nblocks; i++) {
@ -718,6 +760,9 @@ newliveness(Node *fn, Prog *ptxt, Array *cfg, Array *vars, int computedead)
result->varkill[i] = bvalloc(nvars); result->varkill[i] = bvalloc(nvars);
result->livein[i] = bvalloc(nvars); result->livein[i] = bvalloc(nvars);
result->liveout[i] = bvalloc(nvars); result->liveout[i] = bvalloc(nvars);
result->avarinit[i] = bvalloc(nvars);
result->avarinitany[i] = bvalloc(nvars);
result->avarinitall[i] = bvalloc(nvars);
} }
result->livepointers = arraynew(0, sizeof(Bvec*)); result->livepointers = arraynew(0, sizeof(Bvec*));
@ -764,24 +809,32 @@ freeliveness(Liveness *lv)
free(lv->varkill[i]); free(lv->varkill[i]);
free(lv->livein[i]); free(lv->livein[i]);
free(lv->liveout[i]); free(lv->liveout[i]);
free(lv->avarinit[i]);
free(lv->avarinitany[i]);
free(lv->avarinitall[i]);
} }
free(lv->uevar); free(lv->uevar);
free(lv->varkill); free(lv->varkill);
free(lv->livein); free(lv->livein);
free(lv->liveout); free(lv->liveout);
free(lv->avarinit);
free(lv->avarinitany);
free(lv->avarinitall);
free(lv); free(lv);
} }
static void static void
printeffects(Prog *p, Bvec *uevar, Bvec *varkill) printeffects(Prog *p, Bvec *uevar, Bvec *varkill, Bvec *avarinit)
{ {
print("effects of %P", p); print("effects of %P", p);
print("\nuevar: "); print("\nuevar: ");
bvprint(uevar); bvprint(uevar);
print("\nvarkill: "); print("\nvarkill: ");
bvprint(varkill); bvprint(varkill);
print("\navarinit: ");
bvprint(avarinit);
print("\n"); print("\n");
} }
@ -844,6 +897,9 @@ livenessprintblock(Liveness *lv, BasicBlock *bb)
printvars("\tvarkill", lv->varkill[bb->rpo], lv->vars); printvars("\tvarkill", lv->varkill[bb->rpo], lv->vars);
printvars("\tlivein", lv->livein[bb->rpo], lv->vars); printvars("\tlivein", lv->livein[bb->rpo], lv->vars);
printvars("\tliveout", lv->liveout[bb->rpo], lv->vars); printvars("\tliveout", lv->liveout[bb->rpo], lv->vars);
printvars("\tavarinit", lv->avarinit[bb->rpo], lv->vars);
printvars("\tavarinitany", lv->avarinitany[bb->rpo], lv->vars);
printvars("\tavarinitall", lv->avarinitall[bb->rpo], lv->vars);
print("\tprog:\n"); print("\tprog:\n");
for(prog = bb->first;; prog = prog->link) { for(prog = bb->first;; prog = prog->link) {
@ -1118,10 +1174,11 @@ twobitlivepointermap(Liveness *lv, Bvec *liveout, Array *vars, Bvec *args, Bvec
break; break;
} }
} }
// In various and obscure circumstances, such as methods with an unused
// receiver, the this argument and in arguments are omitted from the // The node list only contains declared names.
// node list. We must explicitly preserve these values to ensure that // If the receiver or arguments are unnamed, they will be omitted
// the addresses printed in backtraces are valid. // from the list above. Preserve those values - even though they are unused -
// in order to keep their addresses live for use in stack traces.
thisargtype = getthisx(lv->fn->type); thisargtype = getthisx(lv->fn->type);
if(thisargtype != nil) { if(thisargtype != nil) {
xoffset = 0; xoffset = 0;
@ -1199,8 +1256,7 @@ unlinkedprog(int as)
static Prog* static Prog*
newpcdataprog(Prog *prog, int32 index) newpcdataprog(Prog *prog, int32 index)
{ {
Node from; Node from, to;
Node to;
Prog *pcdata; Prog *pcdata;
nodconst(&from, types[TINT32], PCDATA_StackMapIndex); nodconst(&from, types[TINT32], PCDATA_StackMapIndex);
@ -1227,8 +1283,7 @@ static void
livenessprologue(Liveness *lv) livenessprologue(Liveness *lv)
{ {
BasicBlock *bb; BasicBlock *bb;
Bvec *uevar; Bvec *uevar, *varkill, *avarinit;
Bvec *varkill;
Prog *prog; Prog *prog;
int32 i; int32 i;
int32 nvars; int32 nvars;
@ -1236,39 +1291,86 @@ livenessprologue(Liveness *lv)
nvars = arraylength(lv->vars); nvars = arraylength(lv->vars);
uevar = bvalloc(nvars); uevar = bvalloc(nvars);
varkill = bvalloc(nvars); varkill = bvalloc(nvars);
avarinit = bvalloc(nvars);
for(i = 0; i < arraylength(lv->cfg); i++) { for(i = 0; i < arraylength(lv->cfg); i++) {
bb = *(BasicBlock**)arrayget(lv->cfg, i); bb = *(BasicBlock**)arrayget(lv->cfg, i);
// Walk the block instructions backward and update the block // Walk the block instructions backward and update the block
// effects with the each prog effects. // effects with the each prog effects.
for(prog = bb->last; prog != nil; prog = prog->opt) { for(prog = bb->last; prog != nil; prog = prog->opt) {
progeffects(prog, lv->vars, uevar, varkill); progeffects(prog, lv->vars, uevar, varkill, avarinit);
if(0) printeffects(prog, uevar, varkill); if(debuglive >= 3)
printeffects(prog, uevar, varkill, avarinit);
bvor(lv->varkill[i], lv->varkill[i], varkill); bvor(lv->varkill[i], lv->varkill[i], varkill);
bvandnot(lv->uevar[i], lv->uevar[i], varkill); bvandnot(lv->uevar[i], lv->uevar[i], varkill);
bvor(lv->uevar[i], lv->uevar[i], uevar); bvor(lv->uevar[i], lv->uevar[i], uevar);
bvor(lv->avarinit[i], lv->avarinit[i], avarinit);
} }
} }
free(uevar); free(uevar);
free(varkill); free(varkill);
free(avarinit);
} }
// Solve the liveness dataflow equations. // Solve the liveness dataflow equations.
static void static void
livenesssolve(Liveness *lv) livenesssolve(Liveness *lv)
{ {
BasicBlock *bb; BasicBlock *bb, *succ, *pred;
BasicBlock *succ; Bvec *newlivein, *newliveout, *any, *all;
Bvec *newlivein; int32 rpo, i, j, change;
Bvec *newliveout;
int32 rpo;
int32 i;
int32 j;
int change;
// These temporary bitvectors exist to avoid successive allocations and // These temporary bitvectors exist to avoid successive allocations and
// frees within the loop. // frees within the loop.
newlivein = bvalloc(arraylength(lv->vars)); newlivein = bvalloc(arraylength(lv->vars));
newliveout = bvalloc(arraylength(lv->vars)); newliveout = bvalloc(arraylength(lv->vars));
any = bvalloc(arraylength(lv->vars));
all = bvalloc(arraylength(lv->vars));
// Push avarinitall, avarinitany forward.
// avarinitall says the addressed var is initialized along all paths reaching the block exit.
// avarinitany says the addressed var is initialized along some path reaching the block exit.
for(i = 0; i < arraylength(lv->cfg); i++) {
bb = *(BasicBlock**)arrayget(lv->cfg, i);
rpo = bb->rpo;
if(i == 0)
bvcopy(lv->avarinitall[rpo], lv->avarinit[rpo]);
else {
bvresetall(lv->avarinitall[rpo]);
bvnot(lv->avarinitall[rpo]);
}
bvcopy(lv->avarinitany[rpo], lv->avarinit[rpo]);
}
change = 1;
while(change != 0) {
change = 0;
for(i = 0; i < arraylength(lv->cfg); i++) {
bb = *(BasicBlock**)arrayget(lv->cfg, i);
rpo = bb->rpo;
bvresetall(any);
bvresetall(all);
for(j = 0; j < arraylength(bb->pred); j++) {
pred = *(BasicBlock**)arrayget(bb->pred, j);
if(j == 0) {
bvcopy(any, lv->avarinitany[pred->rpo]);
bvcopy(all, lv->avarinitall[pred->rpo]);
} else {
bvor(any, any, lv->avarinitany[pred->rpo]);
bvand(all, all, lv->avarinitall[pred->rpo]);
}
}
bvor(any, any, lv->avarinit[rpo]);
bvor(all, all, lv->avarinit[rpo]);
if(bvcmp(any, lv->avarinitany[rpo])) {
change = 1;
bvcopy(lv->avarinitany[rpo], any);
}
if(bvcmp(all, lv->avarinitall[rpo])) {
change = 1;
bvcopy(lv->avarinitall[rpo], all);
}
}
}
// Iterate through the blocks in reverse round-robin fashion. A work // Iterate through the blocks in reverse round-robin fashion. A work
// queue might be slightly faster. As is, the number of iterations is // queue might be slightly faster. As is, the number of iterations is
@ -1279,6 +1381,9 @@ livenesssolve(Liveness *lv)
// Walk blocks in the general direction of propagation. This // Walk blocks in the general direction of propagation. This
// improves convergence. // improves convergence.
for(i = arraylength(lv->cfg) - 1; i >= 0; i--) { for(i = arraylength(lv->cfg) - 1; i >= 0; i--) {
// A variable is live on output from this block
// if it is live on input to some successor.
//
// out[b] = \bigcup_{s \in succ[b]} in[s] // out[b] = \bigcup_{s \in succ[b]} in[s]
bb = *(BasicBlock**)arrayget(lv->cfg, i); bb = *(BasicBlock**)arrayget(lv->cfg, i);
rpo = bb->rpo; rpo = bb->rpo;
@ -1291,6 +1396,11 @@ livenesssolve(Liveness *lv)
change = 1; change = 1;
bvcopy(lv->liveout[rpo], newliveout); bvcopy(lv->liveout[rpo], newliveout);
} }
// A variable is live on input to this block
// if it is live on output from this block and
// not set by the code in this block.
//
// in[b] = uevar[b] \cup (out[b] \setminus varkill[b]) // in[b] = uevar[b] \cup (out[b] \setminus varkill[b])
bvandnot(newlivein, lv->liveout[rpo], lv->varkill[rpo]); bvandnot(newlivein, lv->liveout[rpo], lv->varkill[rpo]);
bvor(lv->livein[rpo], newlivein, lv->uevar[rpo]); bvor(lv->livein[rpo], newlivein, lv->uevar[rpo]);
@ -1299,6 +1409,31 @@ livenesssolve(Liveness *lv)
free(newlivein); free(newlivein);
free(newliveout); free(newliveout);
free(any);
free(all);
}
// This function is slow but it is only used for generating debug prints.
// Check whether n is marked live in args/locals.
static int
islive(Node *n, Bvec *args, Bvec *locals)
{
int i;
switch(n->class) {
case PPARAM:
case PPARAMOUT:
for(i = 0; i < n->type->width/widthptr*BitsPerPointer; i++)
if(bvget(args, n->xoffset/widthptr*BitsPerPointer + i))
return 1;
break;
case PAUTO:
for(i = 0; i < n->type->width/widthptr*BitsPerPointer; i++)
if(bvget(locals, (n->xoffset + stkptrsize)/widthptr*BitsPerPointer + i))
return 1;
break;
}
return 0;
} }
// Visits all instructions in a basic block and computes a bit vector of live // Visits all instructions in a basic block and computes a bit vector of live
@ -1306,10 +1441,11 @@ livenesssolve(Liveness *lv)
static void static void
livenessepilogue(Liveness *lv) livenessepilogue(Liveness *lv)
{ {
BasicBlock *bb; BasicBlock *bb, *pred;
Bvec *livein, *liveout, *uevar, *varkill, *args, *locals; Bvec *livein, *liveout, *uevar, *varkill, *args, *locals, *avarinit, *any, *all;
Node *n;
Prog *p, *next; Prog *p, *next;
int32 i, j, nmsg, nvars, pos; int32 i, j, numlive, startmsg, nmsg, nvars, pos;
char **msg; char **msg;
Fmt fmt; Fmt fmt;
@ -1318,38 +1454,97 @@ livenessepilogue(Liveness *lv)
liveout = bvalloc(nvars); liveout = bvalloc(nvars);
uevar = bvalloc(nvars); uevar = bvalloc(nvars);
varkill = bvalloc(nvars); varkill = bvalloc(nvars);
avarinit = bvalloc(nvars);
any = bvalloc(nvars);
all = bvalloc(nvars);
msg = nil; msg = nil;
nmsg = 0; nmsg = 0;
startmsg = 0;
for(i = 0; i < arraylength(lv->cfg); i++) { for(i = 0; i < arraylength(lv->cfg); i++) {
bb = *(BasicBlock**)arrayget(lv->cfg, i); bb = *(BasicBlock**)arrayget(lv->cfg, i);
bvcopy(livein, lv->liveout[bb->rpo]);
// Walk forward through the basic block instructions and // Compute avarinitany and avarinitall for entry to block.
// allocate and empty map for those instructions that need them // This duplicates information known during livenesssolve
for(p = bb->last; p != nil; p = p->opt) { // but avoids storing two more vectors for each block.
if(!issafepoint(p)) bvresetall(any);
continue; bvresetall(all);
for(j = 0; j < arraylength(bb->pred); j++) {
// Allocate a bit vector for each class and facet of pred = *(BasicBlock**)arrayget(bb->pred, j);
// value we are tracking. if(j == 0) {
bvcopy(any, lv->avarinitany[pred->rpo]);
// Live stuff first. bvcopy(all, lv->avarinitall[pred->rpo]);
args = bvalloc(argswords() * BitsPerPointer); } else {
arrayadd(lv->argslivepointers, &args); bvor(any, any, lv->avarinitany[pred->rpo]);
locals = bvalloc(localswords() * BitsPerPointer); bvand(all, all, lv->avarinitall[pred->rpo]);
arrayadd(lv->livepointers, &locals);
// Dead stuff second.
if(lv->deadvalues != nil) {
args = bvalloc(argswords() * BitsPerPointer);
arrayadd(lv->argsdeadvalues, &args);
locals = bvalloc(localswords() * BitsPerPointer);
arrayadd(lv->deadvalues, &locals);
} }
} }
// Walk forward through the basic block instructions and
// allocate liveness maps for those instructions that need them.
// Seed the maps with information about the addrtaken variables.
for(p = bb->first;; p = p->link) {
progeffects(p, lv->vars, uevar, varkill, avarinit);
bvor(any, any, avarinit);
bvor(all, all, avarinit);
if(issafepoint(p)) {
if(debuglive >= 3) {
// Diagnose ambiguously live variables (any &^ all).
// livein and liveout are dead here and used as temporaries.
bvresetall(livein);
bvandnot(liveout, any, all);
if(bvcmp(livein, liveout) != 0) {
for(pos = 0; pos < liveout->n; pos++) {
if(bvget(liveout, pos)) {
n = *(Node**)arrayget(lv->vars, pos);
if(!n->diag && strncmp(n->sym->name, "autotmp_", 8) != 0) {
n->diag = 1;
warnl(p->lineno, "%N: %lN is ambiguously live", curfn->nname, n);
}
}
bvset(all, pos); // silence future warnings in this block
}
}
}
// Allocate a bit vector for each class and facet of
// value we are tracking.
// Live stuff first.
args = bvalloc(argswords() * BitsPerPointer);
arrayadd(lv->argslivepointers, &args);
locals = bvalloc(localswords() * BitsPerPointer);
arrayadd(lv->livepointers, &locals);
if(debuglive >= 3) {
print("%P\n", p);
printvars("avarinitany", any, lv->vars);
}
// Record any values with an "address taken" reaching
// this code position as live. Must do now instead of below
// because the any/all calculation requires walking forward
// over the block (as this loop does), while the liveout
// requires walking backward (as the next loop does).
twobitlivepointermap(lv, any, lv->vars, args, locals);
// Dead stuff second.
if(lv->deadvalues != nil) {
args = bvalloc(argswords() * BitsPerPointer);
arrayadd(lv->argsdeadvalues, &args);
locals = bvalloc(localswords() * BitsPerPointer);
arrayadd(lv->deadvalues, &locals);
}
}
if(p == bb->last)
break;
}
if(debuglive) { if(debuglive >= 1 && strcmp(curfn->nname->sym->name, "init") != 0) {
nmsg = arraylength(lv->livepointers); nmsg = arraylength(lv->livepointers);
startmsg = nmsg;
msg = xmalloc(nmsg*sizeof msg[0]); msg = xmalloc(nmsg*sizeof msg[0]);
for(j=0; j<nmsg; j++) for(j=0; j<nmsg; j++)
msg[j] = nil; msg[j] = nil;
@ -1363,14 +1558,15 @@ livenessepilogue(Liveness *lv)
fatal("livenessepilogue"); fatal("livenessepilogue");
} }
bvcopy(livein, lv->liveout[bb->rpo]);
for(p = bb->last; p != nil; p = next) { for(p = bb->last; p != nil; p = next) {
next = p->opt; // splicebefore modifies p->opt next = p->opt; // splicebefore modifies p->opt
// Propagate liveness information // Propagate liveness information
progeffects(p, lv->vars, uevar, varkill); progeffects(p, lv->vars, uevar, varkill, avarinit);
bvcopy(liveout, livein); bvcopy(liveout, livein);
bvandnot(livein, liveout, varkill); bvandnot(livein, liveout, varkill);
bvor(livein, livein, uevar); bvor(livein, livein, uevar);
if(printnoise){ if(debuglive >= 3 && issafepoint(p)){
print("%P\n", p); print("%P\n", p);
printvars("uevar", uevar, lv->vars); printvars("uevar", uevar, lv->vars);
printvars("varkill", varkill, lv->vars); printvars("varkill", varkill, lv->vars);
@ -1381,25 +1577,37 @@ livenessepilogue(Liveness *lv)
// Found an interesting instruction, record the // Found an interesting instruction, record the
// corresponding liveness information. // corresponding liveness information.
if(debuglive) {
fmtstrinit(&fmt);
fmtprint(&fmt, "%L: live at ", p->lineno);
if(p->as == ACALL)
fmtprint(&fmt, "CALL %lS:", p->to.sym);
else
fmtprint(&fmt, "TEXT %lS:", p->from.sym);
for(j = 0; j < arraylength(lv->vars); j++)
if(bvget(liveout, j))
fmtprint(&fmt, " %N", *(Node**)arrayget(lv->vars, j));
fmtprint(&fmt, "\n");
msg[pos] = fmtstrflush(&fmt);
}
// Record live pointers. // Record live pointers.
args = *(Bvec**)arrayget(lv->argslivepointers, pos); args = *(Bvec**)arrayget(lv->argslivepointers, pos);
locals = *(Bvec**)arrayget(lv->livepointers, pos); locals = *(Bvec**)arrayget(lv->livepointers, pos);
twobitlivepointermap(lv, liveout, lv->vars, args, locals); twobitlivepointermap(lv, liveout, lv->vars, args, locals);
// Show live pointer bitmaps.
// We're interpreting the args and locals bitmap instead of liveout so that we
// include the bits added by the avarinit logic in the
// previous loop.
if(debuglive >= 1) {
fmtstrinit(&fmt);
fmtprint(&fmt, "%L: live at ", p->lineno);
if(p->as == ACALL)
fmtprint(&fmt, "call to %s:", p->to.node->sym->name);
else
fmtprint(&fmt, "entry to %s:", p->from.node->sym->name);
numlive = 0;
for(j = 0; j < arraylength(lv->vars); j++) {
n = *(Node**)arrayget(lv->vars, j);
if(islive(n, args, locals)) {
fmtprint(&fmt, " %N", n);
numlive++;
}
}
fmtprint(&fmt, "\n");
if(numlive == 0) // squelch message
free(fmtstrflush(&fmt));
else
msg[--startmsg] = fmtstrflush(&fmt);
}
// Record dead values. // Record dead values.
if(lv->deadvalues != nil) { if(lv->deadvalues != nil) {
args = *(Bvec**)arrayget(lv->argsdeadvalues, pos); args = *(Bvec**)arrayget(lv->argsdeadvalues, pos);
@ -1411,17 +1619,12 @@ livenessepilogue(Liveness *lv)
// The TEXT instruction annotation is implicit. // The TEXT instruction annotation is implicit.
if(p->as == ACALL) { if(p->as == ACALL) {
if(isdeferreturn(p)) { if(isdeferreturn(p)) {
// Because this runtime call // runtime.deferreturn modifies its return address to return
// modifies its return address // back to the CALL, not to the subsequent instruction.
// to return back to itself, // Because the return comes back one instruction early,
// emitting a PCDATA before the // the PCDATA must begin one instruction early too.
// call instruction will result // The instruction before a call to deferreturn is always a
// in an off by one error during // no-op, to keep PC-specific data unambiguous.
// a stack walk. Fortunately,
// the compiler inserts a no-op
// instruction before this call
// so we can reliably anchor the
// PCDATA to that instruction.
splicebefore(lv, bb, newpcdataprog(p->opt, pos), p->opt); splicebefore(lv, bb, newpcdataprog(p->opt, pos), p->opt);
} else { } else {
splicebefore(lv, bb, newpcdataprog(p, pos), p); splicebefore(lv, bb, newpcdataprog(p, pos), p);
@ -1431,13 +1634,14 @@ livenessepilogue(Liveness *lv)
pos--; pos--;
} }
} }
if(debuglive) { if(debuglive >= 1) {
for(j=0; j<nmsg; j++) for(j=startmsg; j<nmsg; j++)
if(msg[j] != nil) if(msg[j] != nil)
print("%s", msg[j]); print("%s", msg[j]);
free(msg); free(msg);
msg = nil; msg = nil;
nmsg = 0; nmsg = 0;
startmsg = 0;
} }
} }
@ -1445,6 +1649,130 @@ livenessepilogue(Liveness *lv)
free(liveout); free(liveout);
free(uevar); free(uevar);
free(varkill); free(varkill);
free(avarinit);
free(any);
free(all);
}
static int
printbitset(int printed, char *name, Array *vars, Bvec *bits)
{
int i, started;
Node *n;
started = 0;
for(i=0; i<arraylength(vars); i++) {
if(!bvget(bits, i))
continue;
if(!started) {
if(!printed) {
printed = 1;
print("\t");
} else
print(" ");
started = 1;
printed = 1;
print("%s=", name);
} else {
print(",");
}
n = *(Node**)arrayget(vars, i);
print("%s", n->sym->name);
}
return printed;
}
// Prints the computed liveness information and inputs, for debugging.
// This format synthesizes the information used during the multiple passes
// into a single presentation.
static void
livenessprintdebug(Liveness *lv)
{
int i, j, printed, nsafe;
BasicBlock *bb;
Prog *p;
Bvec *uevar, *varkill, *avarinit, *args, *locals;
Node *n;
print("liveness: %s\n", curfn->nname->sym->name);
uevar = bvalloc(arraylength(lv->vars));
varkill = bvalloc(arraylength(lv->vars));
avarinit = bvalloc(arraylength(lv->vars));
nsafe = 0;
for(i = 0; i < arraylength(lv->cfg); i++) {
if(i > 0)
print("\n");
bb = *(BasicBlock**)arrayget(lv->cfg, i);
// bb#0 pred=1,2 succ=3,4
print("bb#%d pred=", i);
for(j = 0; j < arraylength(bb->pred); j++) {
if(j > 0)
print(",");
print("%d", (*(BasicBlock**)arrayget(bb->pred, j))->rpo);
}
print(" succ=");
for(j = 0; j < arraylength(bb->succ); j++) {
if(j > 0)
print(",");
print("%d", (*(BasicBlock**)arrayget(bb->succ, j))->rpo);
}
print("\n");
// initial settings
printed = 0;
printed = printbitset(printed, "uevar", lv->vars, lv->uevar[bb->rpo]);
printed = printbitset(printed, "livein", lv->vars, lv->livein[bb->rpo]);
if(printed)
print("\n");
// program listing, with individual effects listed
for(p = bb->first;; p = p->link) {
print("%P\n", p);
progeffects(p, lv->vars, uevar, varkill, avarinit);
printed = 0;
printed = printbitset(printed, "uevar", lv->vars, uevar);
printed = printbitset(printed, "varkill", lv->vars, varkill);
printed = printbitset(printed, "avarinit", lv->vars, avarinit);
if(printed)
print("\n");
if(issafepoint(p)) {
args = *(Bvec**)arrayget(lv->argslivepointers, nsafe);
locals = *(Bvec**)arrayget(lv->livepointers, nsafe);
nsafe++;
print("\tlive=");
printed = 0;
for(j = 0; j < arraylength(lv->vars); j++) {
n = *(Node**)arrayget(lv->vars, j);
if(islive(n, args, locals)) {
if(printed++)
print(",");
print("%N", n);
}
}
print("\n");
}
if(p == bb->last)
break;
}
// bb bitsets
print("end\n");
printed = printbitset(printed, "varkill", lv->vars, lv->varkill[bb->rpo]);
printed = printbitset(printed, "liveout", lv->vars, lv->liveout[bb->rpo]);
printed = printbitset(printed, "avarinit", lv->vars, lv->avarinit[bb->rpo]);
printed = printbitset(printed, "avarinitany", lv->vars, lv->avarinitany[bb->rpo]);
printed = printbitset(printed, "avarinitall", lv->vars, lv->avarinitall[bb->rpo]);
if(printed)
print("\n");
}
print("\n");
free(uevar);
free(varkill);
free(avarinit);
} }
// Dumps an array of bitmaps to a symbol as a sequence of uint32 values. The // Dumps an array of bitmaps to a symbol as a sequence of uint32 values. The
@ -1456,14 +1784,8 @@ static void
twobitwritesymbol(Array *arr, Sym *sym, Bvec *check) twobitwritesymbol(Array *arr, Sym *sym, Bvec *check)
{ {
Bvec *bv; Bvec *bv;
int off; int off, i, j, len, pos;
uint32 bit; uint32 bit, word, checkword;
uint32 word;
uint32 checkword;
int32 i;
int32 j;
int32 len;
int32 pos;
len = arraylength(arr); len = arraylength(arr);
// Dump the length of the bitmap array. // Dump the length of the bitmap array.
@ -1478,8 +1800,7 @@ twobitwritesymbol(Array *arr, Sym *sym, Bvec *check)
word = bv->b[j/32]; word = bv->b[j/32];
checkword = check->b[j/32]; checkword = check->b[j/32];
if(word != checkword) { if(word != checkword) {
// Found a mismatched word, find // Found a mismatched word; find the mismatched bit.
// the mismatched bit.
for(pos = 0; pos < 32; pos++) { for(pos = 0; pos < 32; pos++) {
bit = 1 << pos; bit = 1 << pos;
if((word & bit) && !(checkword & bit)) { if((word & bit) && !(checkword & bit)) {
@ -1516,26 +1837,40 @@ printprog(Prog *p)
void void
liveness(Node *fn, Prog *firstp, Sym *argssym, Sym *livesym, Sym *deadsym) liveness(Node *fn, Prog *firstp, Sym *argssym, Sym *livesym, Sym *deadsym)
{ {
Array *cfg; Array *cfg, *vars;
Array *vars;
Liveness *lv; Liveness *lv;
int debugdelta;
if(0) print("curfn->nname->sym->name is %s\n", curfn->nname->sym->name); // Change name to dump debugging information only for a specific function.
if(0) printprog(firstp); debugdelta = 0;
if(strcmp(curfn->nname->sym->name, "!") == 0)
debugdelta = 2;
debuglive += debugdelta;
if(debuglive >= 3) {
print("liveness: %s\n", curfn->nname->sym->name);
printprog(firstp);
}
checkptxt(fn, firstp); checkptxt(fn, firstp);
// Construct the global liveness state. // Construct the global liveness state.
cfg = newcfg(firstp); cfg = newcfg(firstp);
if(0) printcfg(cfg); if(debuglive >= 3)
printcfg(cfg);
vars = getvariables(fn, deadsym != nil); vars = getvariables(fn, deadsym != nil);
lv = newliveness(fn, firstp, cfg, vars, deadsym != nil); lv = newliveness(fn, firstp, cfg, vars, deadsym != nil);
// Run the dataflow framework. // Run the dataflow framework.
livenessprologue(lv); livenessprologue(lv);
if(0) livenessprintcfg(lv); if(debuglive >= 3)
livenessprintcfg(lv);
livenesssolve(lv); livenesssolve(lv);
if(0) livenessprintcfg(lv); if(debuglive >= 3)
livenessprintcfg(lv);
livenessepilogue(lv); livenessepilogue(lv);
if(debuglive >= 2)
livenessprintdebug(lv);
// Emit the live pointer map data structures // Emit the live pointer map data structures
twobitwritesymbol(lv->livepointers, livesym, nil); twobitwritesymbol(lv->livepointers, livesym, nil);
@ -1549,4 +1884,6 @@ liveness(Node *fn, Prog *firstp, Sym *argssym, Sym *livesym, Sym *deadsym)
freeliveness(lv); freeliveness(lv);
arrayfree(vars); arrayfree(vars);
freecfg(cfg); freecfg(cfg);
debuglive -= debugdelta;
} }

81
test/live.go Normal file
View File

@ -0,0 +1,81 @@
// errorcheck -0 -l -live
// Copyright 2014 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package main
func f1() {
var x *int
print(&x) // ERROR "live at call to printpointer: x$"
print(&x) // ERROR "live at call to printpointer: x$"
}
func f2(b bool) {
if b {
print(0) // nothing live here
return
}
var x *int
print(&x) // ERROR "live at call to printpointer: x$"
print(&x) // ERROR "live at call to printpointer: x$"
}
func f3(b bool) {
print(0)
if b == false {
print(0) // nothing live here
return
}
if b {
var x *int
print(&x) // ERROR "live at call to printpointer: x$"
print(&x) // ERROR "live at call to printpointer: x$"
} else {
var y *int
print(&y) // ERROR "live at call to printpointer: y$"
print(&y) // ERROR "live at call to printpointer: y$"
}
print(0) // ERROR "live at call to printint: x y$"
}
// The old algorithm treated x as live on all code that
// could flow to a return statement, so it included the
// function entry and code above the declaration of x
// but would not include an indirect use of x in an infinite loop.
// Check that these cases are handled correctly.
func f4(b1, b2 bool) { // x not live here
if b2 {
print(0) // x not live here
return
}
var z **int
x := new(int)
*x = 42
z = &x
print(**z) // ERROR "live at call to printint: x z$"
if b2 {
print(1) // ERROR "live at call to printint: x$"
return
}
for {
print(**z) // ERROR "live at call to printint: x z$"
}
}
func f5(b1 bool) {
var z **int
if b1 {
x := new(int)
*x = 42
z = &x
} else {
y := new(int)
*y = 54
z = &y
}
print(**z) // ERROR "live at call to printint: x y$"
}