cmd/compile: fold redundant zero-extension after signed load on arm64 by gaul · Pull Request #79652 · golang/go

gaul · 2026-05-26T03:43:15Z

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.

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, golang#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.

gopherbot · 2026-05-26T03:51:15Z

This PR (HEAD: f36ef22) has been imported to Gerrit for code review.

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gopherbot · 2026-05-26T04:18:46Z

Message from Gopher Robot:

Patch Set 1:

(1 comment)