On Skylake server, _dl_runtime_resolve_avx512_opt is used to preserve
the first 8 vector registers. The code layout is
if only %xmm0 - %xmm7 registers are used
preserve %xmm0 - %xmm7 registers
if only %ymm0 - %ymm7 registers are used
preserve %ymm0 - %ymm7 registers
preserve %zmm0 - %zmm7 registers
Branch predication always executes the fallthrough code path to preserve
%zmm0 - %zmm7 registers speculatively, even though only %xmm0 - %xmm7
registers are used. This leads to lower CPU frequency on Skylake
server. This patch changes the fallthrough code path to preserve
%xmm0 - %xmm7 registers instead:
if whole %zmm0 - %zmm7 registers are used
preserve %zmm0 - %zmm7 registers
if only %ymm0 - %ymm7 registers are used
preserve %ymm0 - %ymm7 registers
preserve %xmm0 - %xmm7 registers
Tested on Skylake server.
[BZ #21258]
* sysdeps/x86_64/dl-trampoline.S (_dl_runtime_resolve_opt):
Define only if _dl_runtime_resolve is defined to
_dl_runtime_resolve_sse_vex.
* sysdeps/x86_64/dl-trampoline.h (_dl_runtime_resolve_opt):
Fallthrough to _dl_runtime_resolve_sse_vex.
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