cmd/compile: reduce bits.Div64(0, lo, y) to 64 bit division

With this change, these two functions generate identical code: func f(x uint64) (uint64, uint64) { return bits.Div64(0, x, 5) } func g(x uint64) (uint64, uint64) { return x / 5, x % 5 } Updates #31582 Change-Id: Ia96c2e67f8af5dd985823afee5f155608c04a4b6 Reviewed-on: https://go-review.googlesource.com/c/go/+/173197 Reviewed-by: Brad Fitzpatrick <[email protected]> Run-TryBot: Brad Fitzpatrick <[email protected]> TryBot-Result: Gobot Gobot <[email protected]>
golang · Apr 20, 2019 · 68d4b12 · 68d4b12
1 parent d23cba6
commit 68d4b12
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diff --git a/src/cmd/compile/internal/ssa/gen/generic.rules b/src/cmd/compile/internal/ssa/gen/generic.rules
@@ -146,6 +146,8 @@
 (Div64u (Const64 [c])  (Const64 [d])) && d != 0 -> (Const64 [int64(uint64(c)/uint64(d))])
 (Div32F (Const32F [c]) (Const32F [d])) -> (Const32F [auxFrom32F(auxTo32F(c) / auxTo32F(d))])
 (Div64F (Const64F [c]) (Const64F [d])) -> (Const64F [auxFrom64F(auxTo64F(c) / auxTo64F(d))])
+(Select0 (Div128u (Const64 [0]) lo y)) -> (Div64u lo y)
+(Select1 (Div128u (Const64 [0]) lo y)) -> (Mod64u lo y)
 
 (Not (ConstBool [c])) -> (ConstBool [1-c])
 

diff --git a/src/cmd/compile/internal/ssa/rewritegeneric.go b/src/cmd/compile/internal/ssa/rewritegeneric.go
diff --git a/test/codegen/mathbits.go b/test/codegen/mathbits.go
@@ -538,3 +538,8 @@ func Div64(hi, lo, x uint64) (q, r uint64) {
 	// amd64:"DIVQ"
 	return bits.Div64(hi, lo, x)
 }
+
+func Div64degenerate(x uint64) (q, r uint64) {
+	// amd64:-"DIVQ"
+	return bits.Div64(0, x, 5)
+}