From 478277f81283b9e941c4fdadc253797e6d035971 Mon Sep 17 00:00:00 2001 From: Austin Clements Date: Tue, 23 Feb 2021 22:03:29 -0500 Subject: [PATCH] cmd/compile/internal-abi: use x87 mode, not MMX mode Florian Weimer pointed out that my justification for using MMX mode was nonsense and that staying in x87 mode simplifies transitions to and from C. Hence, switch the spec to say we're always in x87 mode. For #40724. Change-Id: Iad916b2c376db41f95614aa6897f6b1184576bb7 Reviewed-on: https://go-review.googlesource.com/c/go/+/295789 Trust: Austin Clements Reviewed-by: Cherry Zhang Reviewed-by: Than McIntosh --- src/cmd/compile/internal-abi.md | 11 +++++------ 1 file changed, 5 insertions(+), 6 deletions(-) diff --git a/src/cmd/compile/internal-abi.md b/src/cmd/compile/internal-abi.md index f4ef2cc869..3a3509d8c2 100644 --- a/src/cmd/compile/internal-abi.md +++ b/src/cmd/compile/internal-abi.md @@ -455,13 +455,12 @@ The arithmetic status flags are treated like scratch registers and not preserved across calls. All other bits in RFLAGS are system flags. -The CPU is always in MMX technology state (not x87 mode). +At function calls and returns, the CPU is in x87 mode (not MMX +technology mode). -*Rationale*: Go on amd64 uses the XMM registers and never uses the x87 -registers, so it makes sense to assume the CPU is in MMX mode. -Otherwise, any function that used the XMM registers would have to -execute an EMMS instruction before calling another function or -returning (this is the case in the SysV ABI). +*Rationale*: Go on amd64 does not use either the x87 registers or MMX +registers. Hence, we follow the SysV platform conventions in order to +simplify transitions to and from the C ABI. At calls, the MXCSR control bits are always set as follows: