[libc][bazel] Enable software prefetching for memcpy #108939
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This will affect only Bazel configuration for now.
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@llvm/pr-subscribers-libc Author: Guillaume Chatelet (gchatelet) ChangesThis will affect only Bazel configuration for now. Full diff: https://github.com/llvm/llvm-project/pull/108939.diff 1 Files Affected:
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 f65da9e98226b6..96d7fa86e9ddf2 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
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michaelrj-google
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Sep 17, 2024
gchatelet
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Oct 7, 2024
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`.
gchatelet
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Oct 21, 2024
…movsb` When using `-mprefer-vector-width=128` with `-march=sandybridge` copying 3 cache lines in one go (192B) gets converted into `rep;movsb` which translate into a 60% hit in performance. Consecutive calls to `__builtin_memcpy_inline` (implementation behind `builtin::Memcpy::block_offset`) are not coalesced by the compiler and so calling it three times in a row generates the desired assembly. It only differs in the interleaving of the loads and stores and does not affect performance. This is needed to reland llvm#108939.
gchatelet
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Oct 22, 2024
…movsb` (#113161) When using `-mprefer-vector-width=128` with `-march=sandybridge` copying 3 cache lines in one go (192B) gets converted into `rep;movsb` which translate into a 60% hit in performance. Consecutive calls to `__builtin_memcpy_inline` (implementation behind `builtin::Memcpy::block_offset`) are not coalesced by the compiler and so calling it three times in a row generates the desired assembly. It only differs in the interleaving of the loads and stores and does not affect performance. This is needed to reland #108939.
gchatelet
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Oct 28, 2024
This will affect only Bazel configuration for now. This is a reland of llvm#108939 which has been reverted because of codegen issues fixed by llvm#113161.
gchatelet
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NoumanAmir657
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Nov 4, 2024
…3886) This will affect only Bazel configuration for now. This is a reland of llvm#108939 which has been reverted because of codegen issues fixed by llvm#113161.
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This will affect only Bazel configuration for now.