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cmd/compile/internal/ssa: more constant folding rules for ARM

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(ADDconst [c] x) && !isARMImmRot(uint32(c)) && isARMImmRot(uint32(-c)) -> (SUBconst [int64(int32(-c))] x)
(SUBconst [c] x) && !isARMImmRot(uint32(c)) && isARMImmRot(uint32(-c)) -> (ADDconst [int64(int32(-c))] x)
Currently
a = a + 0xfffffff1 is compiled to （variable a is in R0)
MVN $14, R11
ADD R11, R0, R0
After applying the above 2 rules, it becomes
SUB $15, R0, R0

(BICconst [c] (BICconst [d] x)) -> (BICconst [int64(int32(c|d))] x)
This rule also optimizes the generated ARM code.

The other rules are added to avoid to generate less optimized ARM code
when substitutions ADD->SUB happen.

Change-Id: I3ead9aae2b446b674e2ab42d37259d38ceb93a4d
Reviewed-on: https://go-review.googlesource.com/41679
Reviewed-by: Keith Randall <khr@golang.org>
This commit is contained in:
Ben Shi 2017-04-25 10:53:10 +00:00 • committed by Keith Randall
parent c4335f81a2
commit 38fbada557
3 changed files with 340 additions and 0 deletions
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12
src/cmd/compile/internal/ssa/rewrite.go
View file

@ -624,3 +624,15 @@ func reciprocalExact32(c float32) bool {
return true
}
}
// check if an immediate can be directly encoded into an ARM's instruction
func isARMImmRot(v uint32) bool {
for i := 0; i < 16; i++ {
if v&^0xff == 0 {
return true
}
v = v<<2 | v>>30
}
return false
}