cmd/compile: fold redundant zero-extension after signed load on arm64#10
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hbrooks wants to merge 1 commit into
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cmd/compile: fold redundant zero-extension after signed load on arm64#10hbrooks wants to merge 1 commit into
hbrooks wants to merge 1 commit into
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When a sign-extending load (LDRSB/LDRSH/LDRSW) is followed by a single
zero-extension, the upper bits produced by the load are immediately
overwritten and the extension is dead. The load can be switched to its
unsigned form (LDRB/LDRH/LDR) and the extension elided.
For example, code like
func F(s []int8, i int) bool { return s[i] == 0x5d }
previously generated
ldrsb x4, [x1, x3]
ubfx x4, x4, #0, #8
cmp w4, #0x5d
and now generates
ldrb w4, [x1, x3]
cmp w4, #0x5d
RISCV64 (RISCV64.rules) and MIPS (MIPS.rules) already had the equivalent
rewrite; ARM64 was missing it, including the indexed (loadidx) variants.
Found via armlint: LDRSx + zero-extension findings drop from 40 -> 1 on
gofmt and 547 -> 8 on cmd/go. Text section shrinks by 144 bytes
(0.0121%) on gofmt and 2416 bytes (0.0363%) on cmd/go.
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Originally PR golang#79652 in golang/go by @gaul