cmd/compile: invalidate zero-use values during rewrite
This helps remove uses that aren't needed any more.
That in turn helps other rules with Uses==1 conditions fire.
Update #39918
Change-Id: I68635b675472f1d59e59604e4d34b949a0016533
Reviewed-on: https://go-review.googlesource.com/c/go/+/249463
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
diff --git a/src/cmd/compile/internal/ssa/decompose.go b/src/cmd/compile/internal/ssa/decompose.go
index 6e72e38..ab27ba8 100644
--- a/src/cmd/compile/internal/ssa/decompose.go
+++ b/src/cmd/compile/internal/ssa/decompose.go
@@ -23,9 +23,11 @@
}
// Decompose other values
- applyRewrite(f, rewriteBlockdec, rewriteValuedec)
+ // Note: deadcode is false because we need to keep the original
+ // values around so the name component resolution below can still work.
+ applyRewrite(f, rewriteBlockdec, rewriteValuedec, leaveDeadValues)
if f.Config.RegSize == 4 {
- applyRewrite(f, rewriteBlockdec64, rewriteValuedec64)
+ applyRewrite(f, rewriteBlockdec64, rewriteValuedec64, leaveDeadValues)
}
// Split up named values into their components.
@@ -215,7 +217,7 @@
}
func decomposeArgs(f *Func) {
- applyRewrite(f, rewriteBlockdecArgs, rewriteValuedecArgs)
+ applyRewrite(f, rewriteBlockdecArgs, rewriteValuedecArgs, removeDeadValues)
}
func decomposeUser(f *Func) {
diff --git a/src/cmd/compile/internal/ssa/lower.go b/src/cmd/compile/internal/ssa/lower.go
index ab0fa80..f332b2e 100644
--- a/src/cmd/compile/internal/ssa/lower.go
+++ b/src/cmd/compile/internal/ssa/lower.go
@@ -7,7 +7,7 @@
// convert to machine-dependent ops
func lower(f *Func) {
// repeat rewrites until we find no more rewrites
- applyRewrite(f, f.Config.lowerBlock, f.Config.lowerValue)
+ applyRewrite(f, f.Config.lowerBlock, f.Config.lowerValue, removeDeadValues)
}
// checkLower checks for unlowered opcodes and fails if we find one.
diff --git a/src/cmd/compile/internal/ssa/opt.go b/src/cmd/compile/internal/ssa/opt.go
index 6e91fd7..128e614 100644
--- a/src/cmd/compile/internal/ssa/opt.go
+++ b/src/cmd/compile/internal/ssa/opt.go
@@ -6,5 +6,5 @@
// machine-independent optimization
func opt(f *Func) {
- applyRewrite(f, rewriteBlockgeneric, rewriteValuegeneric)
+ applyRewrite(f, rewriteBlockgeneric, rewriteValuegeneric, removeDeadValues)
}
diff --git a/src/cmd/compile/internal/ssa/rewrite.go b/src/cmd/compile/internal/ssa/rewrite.go
index fb35691..3df9ad2 100644
--- a/src/cmd/compile/internal/ssa/rewrite.go
+++ b/src/cmd/compile/internal/ssa/rewrite.go
@@ -20,7 +20,15 @@
"path/filepath"
)
-func applyRewrite(f *Func, rb blockRewriter, rv valueRewriter) {
+type deadValueChoice bool
+
+const (
+ leaveDeadValues deadValueChoice = false
+ removeDeadValues = true
+)
+
+// deadcode indicates that rewrite should try to remove any values that become dead.
+func applyRewrite(f *Func, rb blockRewriter, rv valueRewriter, deadcode deadValueChoice) {
// repeat rewrites until we find no more rewrites
pendingLines := f.cachedLineStarts // Holds statement boundaries that need to be moved to a new value/block
pendingLines.clear()
@@ -56,6 +64,18 @@
*v0 = *v
v0.Args = append([]*Value{}, v.Args...) // make a new copy, not aliasing
}
+ if v.Uses == 0 && v.removeable() {
+ if v.Op != OpInvalid && deadcode == removeDeadValues {
+ // Reset any values that are now unused, so that we decrement
+ // the use count of all of its arguments.
+ // Not quite a deadcode pass, because it does not handle cycles.
+ // But it should help Uses==1 rules to fire.
+ v.reset(OpInvalid)
+ change = true
+ }
+ // No point rewriting values which aren't used.
+ continue
+ }
vchange := phielimValue(v)
if vchange && debug > 1 {
diff --git a/src/cmd/compile/internal/ssa/softfloat.go b/src/cmd/compile/internal/ssa/softfloat.go
index 4b578b1..8db4334 100644
--- a/src/cmd/compile/internal/ssa/softfloat.go
+++ b/src/cmd/compile/internal/ssa/softfloat.go
@@ -72,7 +72,7 @@
if newInt64 && f.Config.RegSize == 4 {
// On 32bit arch, decompose Uint64 introduced in the switch above.
decomposeBuiltIn(f)
- applyRewrite(f, rewriteBlockdec64, rewriteValuedec64)
+ applyRewrite(f, rewriteBlockdec64, rewriteValuedec64, removeDeadValues)
}
}
diff --git a/src/cmd/compile/internal/ssa/value.go b/src/cmd/compile/internal/ssa/value.go
index 7ead0ff..7fc3377 100644
--- a/src/cmd/compile/internal/ssa/value.go
+++ b/src/cmd/compile/internal/ssa/value.go
@@ -460,3 +460,23 @@
return v.Op == OpVarDef || v.Op == OpVarKill || v.Op == OpVarLive || v.Op == OpPhi ||
(v.Op == OpFwdRef || v.Op == OpCopy) && v.Type == types.TypeMem
}
+
+// removeable reports whether the value v can be removed from the SSA graph entirely
+// if its use count drops to 0.
+func (v *Value) removeable() bool {
+ if v.Type.IsVoid() {
+ // Void ops, like nil pointer checks, must stay.
+ return false
+ }
+ if v.Type.IsMemory() {
+ // All memory ops aren't needed here, but we do need
+ // to keep calls at least (because they might have
+ // syncronization operations we can't see).
+ return false
+ }
+ if v.Op.HasSideEffects() {
+ // These are mostly synchronization operations.
+ return false
+ }
+ return true
+}
