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[dev.ssa] cmd/compile: respect phi values in tighten

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Given (say)

b1: <- b2 b3
  v1 = Phi <t> v2 v3
b2:
  v2 = ...
b3:
  ...

tighten will move v2 to b1, since it is only used in b1.

This is wrong; v2 needs to be evaluated before entering b1.
Fix it.

Change-Id: I2cc3b30e3ffd221cf594e36cec534dfd9cf3c6a7
Reviewed-on: https://go-review.googlesource.com/13264
Reviewed-by: Keith Randall <khr@golang.org>
This commit is contained in:
Josh Bleecher Snyder 2015-08-05 15:51:05 -07:00
parent f91ff1a509
commit e5fe33e546
1 changed files with 11 additions and 3 deletions
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14
src/cmd/compile/internal/ssa/tighten.go
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@ -8,7 +8,8 @@ package ssa
// This can reduce the amount of register spilling required,
// if it doesn't also create more live values.
// For now, it handles only the trivial case in which a
// Value with one or fewer args is only used in a single Block.
// Value with one or fewer args is only used in a single Block,
// and not in a phi value.
// TODO: Do something smarter.
// A Value can be moved to any block that
// dominates all blocks in which it is used.
@ -17,6 +18,9 @@ func tighten(f *Func) {
// For each value, the number of blocks in which it is used.
uses := make([]int, f.NumValues())
// For each value, whether that value is ever an arg to a phi value.
phi := make([]bool, f.NumValues())
// For each value, one block in which that value is used.
home := make([]*Block, f.NumValues())
@ -28,11 +32,15 @@ func tighten(f *Func) {
for i := range uses {
uses[i] = 0
}
// No need to reset home; any relevant values will be written anew anyway
// No need to reset home; any relevant values will be written anew anyway.
// No need to reset phi; once used in a phi, always used in a phi.
for _, b := range f.Blocks {
for _, v := range b.Values {
for _, w := range v.Args {
if v.Op == OpPhi {
phi[w.ID] = true
}
uses[w.ID]++
home[w.ID] = b
}
@ -49,7 +57,7 @@ func tighten(f *Func) {
if v.Op == OpPhi {
continue
}
if uses[v.ID] == 1 && home[v.ID] != b && len(v.Args) < 2 {
if uses[v.ID] == 1 && !phi[v.ID] && home[v.ID] != b && len(v.Args) < 2 {
// v is used in exactly one block, and it is not b.
// Furthermore, it takes at most one input,
// so moving it will not increase the