cmd/compile: compare size in dead store elimination

Only remove stores that is shadowed by another store with same or
larger size. Normally we don't need this check because we did check
the types, but unsafe pointer casting can get around it.

Fixes #16769.

Change-Id: I3f7c6c57807b590a2f735007dec6c65a4fa01a34
Reviewed-on: https://go-review.googlesource.com/27320
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

1 files changed, 15 insertions, 7 deletions

diff --git a/src/cmd/compile/internal/ssa/deadstore.go b/src/cmd/compile/internal/ssa/deadstore.go
index a1fc5fd0bb..7acc3b09d4 100644
--- a/src/cmd/compile/internal/ssa/deadstore.go
+++ b/src/cmd/compile/internal/ssa/deadstore.go
@@ -14,8 +14,7 @@ func dse(f *Func) {
 	defer f.retSparseSet(loadUse)
 	storeUse := f.newSparseSet(f.NumValues())
 	defer f.retSparseSet(storeUse)
-	shadowed := f.newSparseSet(f.NumValues())
-	defer f.retSparseSet(shadowed)
+	shadowed := newSparseMap(f.NumValues()) // TODO: cache
 	for _, b := range f.Blocks {
 		// Find all the stores in this block. Categorize their uses:
 		//  loadUse contains stores which are used by a subsequent load.
@@ -81,17 +80,23 @@ func dse(f *Func) {
 			shadowed.clear()
 		}
 		if v.Op == OpStore || v.Op == OpZero {
-			if shadowed.contains(v.Args[0].ID) {
+			var sz int64
+			if v.Op == OpStore {
+				sz = v.AuxInt
+			} else { // OpZero
+				sz = SizeAndAlign(v.AuxInt).Size()
+			}
+			if shadowedSize := int64(shadowed.get(v.Args[0].ID)); shadowedSize != -1 && shadowedSize >= sz {
 				// Modify store into a copy
 				if v.Op == OpStore {
 					// store addr value mem
 					v.SetArgs1(v.Args[2])
 				} else {
 					// zero addr mem
-					sz := v.Args[0].Type.ElemType().Size()
-					if SizeAndAlign(v.AuxInt).Size() != sz {
+					typesz := v.Args[0].Type.ElemType().Size()
+					if sz != typesz {
 						f.Fatalf("mismatched zero/store sizes: %d and %d [%s]",
-							v.AuxInt, sz, v.LongString())
+							sz, typesz, v.LongString())
 					}
 					v.SetArgs1(v.Args[1])
 				}
@@ -99,7 +104,10 @@ func dse(f *Func) {
 				v.AuxInt = 0
 				v.Op = OpCopy
 			} else {
-				shadowed.add(v.Args[0].ID)
+				if sz > 0x7fffffff { // work around sparseMap's int32 value type
+					sz = 0x7fffffff
+				}
+				shadowed.set(v.Args[0].ID, int32(sz))
 			}
 		}
 		// walk to previous store