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cmd/compile: int64(uint64 >> x) >= 0 if x > 0

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This rewrite rule triggers only once, in math/big.quotToFloat64,
as part of converting a uint64 to a float64.

Nevertheless, it is cheap; let's add it.

Change-Id: I3ed4a197a559110fec1bc04b3a8abb4c7fcc2c89
Reviewed-on: https://go-review.googlesource.com/c/go/+/167500
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
This commit is contained in:
Josh Bleecher Snyder 2019-03-13 14:36:34 -07:00
parent 61945fc502
commit 66f5d4e035
2 changed files with 31 additions and 0 deletions
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2
src/cmd/compile/internal/ssa/gen/generic.rules
View File

@ -425,6 +425,8 @@
(Geq32 (And32 _ (Const32 [c])) (Const32 [0])) && int32(c) >= 0 -> (ConstBool [1])
(Geq64 (And64 _ (Const64 [c])) (Const64 [0])) && int64(c) >= 0 -> (ConstBool [1])
(Geq64 (Rsh64Ux64 _ (Const64 [c])) (Const64 [0])) && c > 0 -> (ConstBool [1])
(Greater64U (Const64 [c]) (Const64 [d])) -> (ConstBool [b2i(uint64(c) > uint64(d))])
(Greater32U (Const32 [c]) (Const32 [d])) -> (ConstBool [b2i(uint32(c) > uint32(d))])
(Greater16U (Const16 [c]) (Const16 [d])) -> (ConstBool [b2i(uint16(c) > uint16(d))])

29
src/cmd/compile/internal/ssa/rewritegeneric.go
View File

@ -10714,6 +10714,35 @@ func rewriteValuegeneric_OpGeq64_0(v *Value) bool {
v.AuxInt = 1
return true
}
// match: (Geq64 (Rsh64Ux64 _ (Const64 [c])) (Const64 [0]))
// cond: c > 0
// result: (ConstBool [1])
for {
_ = v.Args[1]
v_0 := v.Args[0]
if v_0.Op != OpRsh64Ux64 {
break
}
_ = v_0.Args[1]
v_0_1 := v_0.Args[1]
if v_0_1.Op != OpConst64 {
break
}
c := v_0_1.AuxInt
v_1 := v.Args[1]
if v_1.Op != OpConst64 {
break
}
if v_1.AuxInt != 0 {
break
}
if !(c > 0) {
break
}
v.reset(OpConstBool)
v.AuxInt = 1
return true
}
return false
}
func rewriteValuegeneric_OpGeq64F_0(v *Value) bool {