Revert "[libc][bazel] Enable software prefetching for memcpy"

[gchatelet]

Reverts #108939

When AVX is available but -mprefer-vector-width=128 some of the mov instructions turn into the x86 rep;movsb instruction leading to poor performance on "old" architectures (sandybridge, haswell). The possible solutions are : get rid of the -mprefer-vector-width option or use smaller static copy sizes in inline_memcpy_x86_sse2_ge64_sw_prefetching. Right now a copy size of 3 cache lines (192B) relying exclusively on xmm registers gets turned into rep;movsb.

[llvmbot]

@llvm/pr-subscribers-libc

Author: Guillaume Chatelet (gchatelet)

Changes

Reverts llvm/llvm-project#108939

When AVX is available but -mprefer-vector-width=128 some of the mov instructions turn into the x86 rep;movsb instruction leading to poor performance on "old" architectures (sandybridge, haswell). The possible solutions are : get rid of the -mprefer-vector-width option or use smaller static copy sizes in inline_memcpy_x86_sse2_ge64_sw_prefetching. Right now a copy size of 3 cache lines (192B) relying exclusively on xmm registers gets turned into rep;movsb.

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Full diff: https://github.com/llvm/llvm-project/pull/111370.diff

1 Files Affected:

• (modified) utils/bazel/llvm-project-overlay/libc/libc_configure_options.bzl (+1-1)

diff --git a/utils/bazel/llvm-project-overlay/libc/libc_configure_options.bzl b/utils/bazel/llvm-project-overlay/libc/libc_configure_options.bzl
index 96d7fa86e9ddf2..f65da9e98226b6 100644
--- a/utils/bazel/llvm-project-overlay/libc/libc_configure_options.bzl
+++ b/utils/bazel/llvm-project-overlay/libc/libc_configure_options.bzl
@@ -24,7 +24,7 @@ LIBC_CONFIGURE_OPTIONS = [
     # Documentation in libc/src/string/memory_utils/...
     # "LIBC_COPT_MEMCPY_USE_EMBEDDED_TINY",
     # "LIBC_COPT_MEMCPY_X86_USE_REPMOVSB_FROM_SIZE",
-    "LIBC_COPT_MEMCPY_X86_USE_SOFTWARE_PREFETCHING",
+    # "LIBC_COPT_MEMCPY_X86_USE_SOFTWARE_PREFETCHING",
     "LIBC_COPT_MEMSET_X86_USE_SOFTWARE_PREFETCHING",
 
     # Documentation in libc/docs/dev/printf_behavior.rst