cmd/compile: clarify division bounds check optimization

The name of the function should mention division.
Eliminate double negatives from the comment describing it.

Change-Id: Icef1a5139b3a91b86acb930af97938f5160f7342
Reviewed-on: https://go-review.googlesource.com/c/go/+/217001
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>

4 files changed, 9 insertions, 7 deletions

diff --git a/src/cmd/compile/internal/amd64/ssa.go b/src/cmd/compile/internal/amd64/ssa.go
index 9ee322b389..b6c1039d9e 100644
--- a/src/cmd/compile/internal/amd64/ssa.go
+++ b/src/cmd/compile/internal/amd64/ssa.go
@@ -257,7 +257,7 @@ func ssaGenValue(s *gc.SSAGenState, v *ssa.Value) {
 
 		// CPU faults upon signed overflow, which occurs when the most
 		// negative int is divided by -1. Handle divide by -1 as a special case.
-		if ssa.NeedsFixUp(v) {
+		if ssa.DivisionNeedsFixUp(v) {
 			var c *obj.Prog
 			switch v.Op {
 			case ssa.OpAMD64DIVQ:
diff --git a/src/cmd/compile/internal/ssa/prove.go b/src/cmd/compile/internal/ssa/prove.go
index 774fa94dbc..01825579d9 100644
--- a/src/cmd/compile/internal/ssa/prove.go
+++ b/src/cmd/compile/internal/ssa/prove.go
@@ -1243,9 +1243,11 @@ func simplifyBlock(sdom SparseTree, ft *factsTable, b *Block) {
 			divdLim, divdLimok := ft.limits[divd.ID]
 			if (divrLimok && (divrLim.max < -1 || divrLim.min > -1)) ||
 				(divdLimok && divdLim.min > mostNegativeDividend[v.Op]) {
-				v.AuxInt = 1 // see NeedsFixUp in genericOps - v.AuxInt = 0 means we have not proved
-				// that the divisor is not -1 and the dividend is not the most negative,
-				// so we need to add fix-up code.
+				// See DivisionNeedsFixUp in rewrite.go.
+				// v.AuxInt = 1 means we have proved both that the divisor is not -1
+				// and that the dividend is not the most negative integer,
+				// so we do not need to add fix-up code.
+				v.AuxInt = 1
 				if b.Func.pass.debug > 0 {
 					b.Func.Warnl(v.Pos, "Proved %v does not need fix-up", v.Op)
 				}
diff --git a/src/cmd/compile/internal/ssa/rewrite.go b/src/cmd/compile/internal/ssa/rewrite.go
index 539941001d..ef24dad747 100644
--- a/src/cmd/compile/internal/ssa/rewrite.go
+++ b/src/cmd/compile/internal/ssa/rewrite.go
@@ -480,8 +480,8 @@ func extend32Fto64F(f float32) float64 {
 	return math.Float64frombits(r)
 }
 
-// NeedsFixUp reports whether the division needs fix-up code.
-func NeedsFixUp(v *Value) bool {
+// DivisionNeedsFixUp reports whether the division needs fix-up code.
+func DivisionNeedsFixUp(v *Value) bool {
 	return v.AuxInt == 0
 }
 
diff --git a/src/cmd/compile/internal/x86/ssa.go b/src/cmd/compile/internal/x86/ssa.go
index aa68e9163a..0c7e5bdb97 100644
--- a/src/cmd/compile/internal/x86/ssa.go
+++ b/src/cmd/compile/internal/x86/ssa.go
@@ -199,7 +199,7 @@ func ssaGenValue(s *gc.SSAGenState, v *ssa.Value) {
 		if v.Op == ssa.Op386DIVL || v.Op == ssa.Op386DIVW ||
 			v.Op == ssa.Op386MODL || v.Op == ssa.Op386MODW {
 
-			if ssa.NeedsFixUp(v) {
+			if ssa.DivisionNeedsFixUp(v) {
 				var c *obj.Prog
 				switch v.Op {
 				case ssa.Op386DIVL, ssa.Op386MODL: