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From: Sunil Pandey <skpgkp2@sourceware.org> To: glibc-cvs@sourceware.org Subject: [glibc/release/2.33/master] x86: Add COND_VZEROUPPER that can replace vzeroupper if no `ret` Date: Tue, 19 Jul 2022 05:53:30 +0000 (GMT) [thread overview] Message-ID: <20220719055330.9C5CF38582BB@sourceware.org> (raw) https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=0edbcc85cc7f12366d7f62f8c39ab5b70b4e1787 commit 0edbcc85cc7f12366d7f62f8c39ab5b70b4e1787 Author: Noah Goldstein <goldstein.w.n@gmail.com> Date: Mon Jun 6 21:11:28 2022 -0700 x86: Add COND_VZEROUPPER that can replace vzeroupper if no `ret` The RTM vzeroupper mitigation has no way of replacing inline vzeroupper not before a return. This can be useful when hoisting a vzeroupper to save code size for example: ``` L(foo): cmpl %eax, %edx jz L(bar) tzcntl %eax, %eax addq %rdi, %rax VZEROUPPER_RETURN L(bar): xorl %eax, %eax VZEROUPPER_RETURN ``` Can become: ``` L(foo): COND_VZEROUPPER cmpl %eax, %edx jz L(bar) tzcntl %eax, %eax addq %rdi, %rax ret L(bar): xorl %eax, %eax ret ``` This code does not change any existing functionality. There is no difference in the objdump of libc.so before and after this patch. Reviewed-by: H.J. Lu <hjl.tools@gmail.com> (cherry picked from commit dd5c483b2598f411428df4d8864c15c4b8a3cd68) Diff: --- sysdeps/x86_64/multiarch/avx-rtm-vecs.h | 1 + sysdeps/x86_64/sysdep.h | 18 ++++++++++++++++++ 2 files changed, 19 insertions(+) diff --git a/sysdeps/x86_64/multiarch/avx-rtm-vecs.h b/sysdeps/x86_64/multiarch/avx-rtm-vecs.h index 3f531dd47f..6ca9f5e6ba 100644 --- a/sysdeps/x86_64/multiarch/avx-rtm-vecs.h +++ b/sysdeps/x86_64/multiarch/avx-rtm-vecs.h @@ -20,6 +20,7 @@ #ifndef _AVX_RTM_VECS_H #define _AVX_RTM_VECS_H 1 +#define COND_VZEROUPPER COND_VZEROUPPER_XTEST #define ZERO_UPPER_VEC_REGISTERS_RETURN \ ZERO_UPPER_VEC_REGISTERS_RETURN_XTEST diff --git a/sysdeps/x86_64/sysdep.h b/sysdeps/x86_64/sysdep.h index 7bebdeb210..93e44be22e 100644 --- a/sysdeps/x86_64/sysdep.h +++ b/sysdeps/x86_64/sysdep.h @@ -106,6 +106,24 @@ lose: \ vzeroupper; \ ret +/* Can be used to replace vzeroupper that is not directly before a + return. This is useful when hoisting a vzeroupper from multiple + return paths to decrease the total number of vzerouppers and code + size. */ +#define COND_VZEROUPPER_XTEST \ + xtest; \ + jz 1f; \ + vzeroall; \ + jmp 2f; \ +1: \ + vzeroupper; \ +2: + +/* In RTM define this as COND_VZEROUPPER_XTEST. */ +#ifndef COND_VZEROUPPER +# define COND_VZEROUPPER vzeroupper +#endif + /* Zero upper vector registers and return. */ #ifndef ZERO_UPPER_VEC_REGISTERS_RETURN # define ZERO_UPPER_VEC_REGISTERS_RETURN \
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