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cmd/internal/obj/x86: shorten MOVQ r64, imm32 for positive immediates

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The ymovq ytab listed {Zilo_m, argList{Ys32, Yrl}} (sign-extended C7
form, 7 bytes) ahead of {Ziq_rp, argList{Yi64, Yrl}}. Since oclass
returns Ys32 for any positive int that fits in s32, every such MOVQ
to register matched the 7-byte form even though Ziq_rp already picks
the optimal 5/7/10-byte encoding based on the value.

Drop the Ys32 entry so all MOVQ immediate-to-register routes through
Ziq_rp, which emits:
  - B8+rd imm32        (5 bytes) when v fits in u32 (implicit zero-ext)
  - 48 C7 /0 imm32     (7 bytes) when v is negative s32 (sign-extends)
  - 48 B8+rd imm64    (10 bytes) otherwise

Add amd64enc test cases covering the positive-imm32 path, including
the 0x80000000 boundary where the sign- and zero-extended forms differ.

The sigreturn__sigaction trampoline in sys_linux_amd64.s previously
relied on the 7-byte encoding of "MOVQ \$SYS_rt_sigreturn, AX" because
gdb and libgcc recognize the Linux signal trampoline by exact byte
match against glibc's __restore_rt. Hand-emit the required bytes with
BYTE directives so the shorter encoding doesn't break gdb backtraces
through signal frames (TestGdbCoreSignalBacktrace).

Verified with x86lint: "oversized MOV encoding" findings drop to zero
on gofmt (422 -> 0) and cmd/go (412 -> 0). Text section shrinks by
811 bytes (0.029%) on gofmt and 1502 bytes (0.010%) on cmd/go.

Change-Id: Ia03ec04a1f4cf7416531c9295ab4ec8c2c663467
GitHub-Last-Rev: 588bfad952
GitHub-Pull-Request: golang/go#79445
Reviewed-on: https://go-review.googlesource.com/c/go/+/779060
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Keith Randall <khr@golang.org>
This commit is contained in:
Andrew Gaul 2026-05-18 18:05:54 +00:00 • committed by Gopher Robot
parent c888fd67f0
commit 0db36238c6
3 changed files with 18 additions and 4 deletions
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9
src/cmd/asm/internal/asm/testdata/amd64enc.s vendored
View file

@ -2425,6 +2425,15 @@ TEXT asmtest(SB),DUPOK|NOSPLIT,$0
MOVQ $-249346713, (R11) // 49c703674523f1
MOVQ $-249346713, DX // 48c7c2674523f1
MOVQ $-249346713, R11 // 49c7c3674523f1
// Positive immediates that fit in 32 bits use the shorter 5-byte
// MOV r64, imm32 (zero-extended) form rather than the 7-byte sign-
// extended c7 /0 form. See Ziq_rp in cmd/internal/obj/x86/asm6.go.
MOVQ $305419896, DX // ba78563412
MOVQ $305419896, R11 // 41bb78563412
MOVQ $2147483648, DX // ba00000080
MOVQ $2147483648, R11 // 41bb00000080
MOVQ $4294967295, DX // baffffffff
MOVQ $4294967295, R11 // 41bbffffffff
MOVQ DX, (BX) // 488913
MOVQ R11, (BX) // 4c891b
MOVQ DX, (R11) // 498913

5
src/cmd/internal/obj/x86/asm6.go
View file

@ -484,8 +484,7 @@ var ymovq = []ytab{
// valid only in 64-bit mode, usually with 64-bit prefix
{Zr_m, 1, argList{Yrl, Yml}}, // 0x89
{Zm_r, 1, argList{Yml, Yrl}}, // 0x8b
{Zilo_m, 2, argList{Ys32, Yrl}}, // 32 bit signed 0xc7,(0)
{Ziq_rp, 1, argList{Yi64, Yrl}}, // 0xb8 -- 32/64 bit immediate
{Ziq_rp, 1, argList{Yi64, Yrl}}, // 0xb8 -- 32/64 bit immediate (Ziq_rp picks 5/7/10-byte form)
{Zilo_m, 2, argList{Yi32, Yml}}, // 0xc7,(0)
{Zm_r_xm, 1, argList{Ymm, Ymr}}, // 0x6e MMX MOVD
{Zr_m_xm, 1, argList{Ymr, Ymm}}, // 0x7e MMX MOVD
@ -1225,7 +1224,7 @@ var optab =
{AMOVNTPD, yxr_ml, Pe, opBytes{0x2b}},
{AMOVNTPS, yxr_ml, Pm, opBytes{0x2b}},
{AMOVNTQ, ymr_ml, Pm, opBytes{0xe7}},
{AMOVQ, ymovq, Pw8, opBytes{0x6f, 0x7f, Pf2, 0xd6, Pf3, 0x7e, Pe, 0xd6, 0x89, 0x8b, 0xc7, 00, 0xb8, 0xc7, 00, 0x6e, 0x7e, Pe, 0x6e, Pe, 0x7e, 0}},
{AMOVQ, ymovq, Pw8, opBytes{0x6f, 0x7f, Pf2, 0xd6, Pf3, 0x7e, Pe, 0xd6, 0x89, 0x8b, 0xb8, 0xc7, 00, 0x6e, 0x7e, Pe, 0x6e, Pe, 0x7e, 0}},
{AMOVQOZX, ymrxr, Pf3, opBytes{0xd6, 0x7e}},
{AMOVSB, ynone, Pb, opBytes{0xa4}},
{AMOVSD, yxmov, Pf2, opBytes{0x10, 0x11}},

8
src/runtime/sys_linux_amd64.s
View file

@ -480,7 +480,13 @@ sigtrampnog:
// the name. The gdb source code is:
// https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=gdb/amd64-linux-tdep.c;h=cbbac1a0c64e1deb8181b9d0ff6404e328e2979d#l178
TEXT runtime·sigreturn__sigaction(SB),NOSPLIT,$0
MOVQ $SYS_rt_sigreturn, AX
// The 7-byte sign-extended "48 c7 c0 0f 00 00 00" form of
// "MOVQ $SYS_rt_sigreturn, AX" is required: gdb and libgcc match
// these exact bytes to recognize the signal trampoline. The Go
// assembler now emits the 5-byte zero-extended "b8 0f 00 00 00"
// form for positive imm32, so write the bytes by hand.
BYTE $0x48; BYTE $0xc7; BYTE $0xc0
BYTE $0x0f; BYTE $0x00; BYTE $0x00; BYTE $0x00
SYSCALL
INT $3 // not reached