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cmd/compile: fix bug in dwarf-gen var location generation

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This patch fixes a bug in the SSA back end's DWARF generation code
that determines variable locations / lifetimes.

The code in question was written to handle sequences of initial
pseudo-ops (zero width instructions such as OpPhi, OpArg, etc) in a
basic block, detecting these ops at the start of a block and then
treating the values specially when emitting ranges for the variables
in those values.  The logic in this code wasn't quite correct, meaning
that a flag variable wasn't being set properly to record the presence
of a block of zero-width value-bearing ops, leading to incorrect or
missing DWARF locations for register params.

Also in this patch is a tweak to some sanity-checking code intended to
catch scheduling problems with OpArg/OpPhi etc. The checks need to
allow for the possibility of an Arg op scheduled after a spill of an
incoming register param inserted by the register allocator. Example:

    b1:
      v13 = ArgIntReg <int> {p1+16} [2] : CX
      v14 = ArgIntReg <int> {p2+16} [5] : R8
      v38 = ArgIntReg <int> {p3+16} [8] : R11
      v35 = ArgIntReg <int> {p1+0} [0] : AX
      v15 = StoreReg <int> v35 : .autotmp_4[int]
      v40  = Arg <int> {p4} [16] : p4+16[int]
      v1 = InitMem <mem>
      v3 = SB <uintptr> : SB
      v18 = CMPQ <flags> v14 v13
      NE v18 → b3 b2 (unlikely) (18)

Here the register allocator has decided to spill v35, meaning that the
OpArg v40 is no longer going to be positioned prior to all other
non-zero-width ops; this is a valid scenario and needs to be handled
properly by the debug code.

Fixes #46425.

Change-Id: I239b3ad56a9c1b8ebf68af42e1f57308293ed7e6
Reviewed-on: https://go-review.googlesource.com/c/go/+/332269
Trust: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
This commit is contained in:
Than McIntosh 2021-07-01 11:22:02 -04:00
parent 770899f7e1
commit ef8ae82b37
1 changed files with 13 additions and 6 deletions
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17
src/cmd/compile/internal/ssa/debug.go
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@ -1115,7 +1115,13 @@ func (state *debugState) buildLocationLists(blockLocs []*BlockDebug) {
continue
}
mustBeFirst := func(v *Value) bool {
return v.Op == OpPhi || v.Op.isLoweredGetClosurePtr() ||
v.Op == OpArgIntReg || v.Op == OpArgFloatReg
}
zeroWidthPending := false
blockPrologComplete := false // set to true at first non-zero-width op
apcChangedSize := 0 // size of changedVars for leading Args, Phi, ClosurePtr
// expect to see values in pattern (apc)* (zerowidth|real)*
for _, v := range b.Values {
@ -1125,16 +1131,16 @@ func (state *debugState) buildLocationLists(blockLocs []*BlockDebug) {
if opcodeTable[v.Op].zeroWidth {
if changed {
if hasAnyArgOp(v) || v.Op == OpPhi || v.Op.isLoweredGetClosurePtr() {
if mustBeFirst(v) || v.Op == OpArg {
// These ranges begin at true beginning of block, not after first instruction
if zeroWidthPending {
panic(fmt.Errorf("Unexpected op '%s' mixed with OpArg/OpPhi/OpLoweredGetClosurePtr at beginning of block %s in %s\n%s", v.LongString(), b, b.Func.Name, b.Func))
if blockPrologComplete && mustBeFirst(v) {
panic(fmt.Errorf("Unexpected placement of op '%s' appearing after non-pseudo-op at beginning of block %s in %s\n%s", v.LongString(), b, b.Func.Name, b.Func))
}
apcChangedSize = len(state.changedVars.contents())
continue
}
// Other zero-width ops must wait on a "real" op.
zeroWidthPending = true
continue
}
}
continue
}
@ -1145,6 +1151,7 @@ func (state *debugState) buildLocationLists(blockLocs []*BlockDebug) {
// Not zero-width; i.e., a "real" instruction.
zeroWidthPending = false
blockPrologComplete = true
for i, varID := range state.changedVars.contents() {
if i < apcChangedSize { // buffered true start-of-block changes
state.updateVar(VarID(varID), v.Block, BlockStart)