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make fast unaligned memory accesses implicit with SSE4.2 or SSE4a

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This is a follow-on from the discussion in http://reviews.llvm.org/D12154.

This change allows memset/memcpy to use SSE or AVX memory accesses for any chip that has
generally fast unaligned memory ops.

A motivating use case for this change is a clang invocation that doesn't explicitly set
the CPU, but does target a feature that we know only exists on a CPU that supports fast
unaligned memops. For example:
$ clang -O1 foo.c -mavx

This resolves a difference in lowering noted in PR24449:
https://llvm.org/bugs/show_bug.cgi?id=24449

Before this patch, we used different store types depending on whether the example can be
lowered as a memset or not.

Differential Revision: http://reviews.llvm.org/D12288

llvm-svn: 245950
This commit is contained in:
Sanjay Patel 2015-08-25 16:29:21 +00:00
parent a1bc27c2d3
commit 7089a6d8e2
2 changed files with 12 additions and 0 deletions
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7
lib/Target/X86/X86Subtarget.cpp
View File

@ -192,6 +192,13 @@ void X86Subtarget::initSubtargetFeatures(StringRef CPU, StringRef FS) {
// Parse features string and set the CPU.
ParseSubtargetFeatures(CPUName, FullFS);
// All CPUs that implement SSE4.2 or SSE4A support unaligned accesses of
// 16-bytes and under that are reasonably fast. These features were
// introduced with Intel's Nehalem/Silvermont and AMD's Family10h
// micro-architectures respectively.
if (hasSSE42() || hasSSE4A())
IsUAMemUnder32Slow = false;
InstrItins = getInstrItineraryForCPU(CPUName);
// It's important to keep the MCSubtargetInfo feature bits in sync with

5
test/CodeGen/X86/slow-unaligned-mem.ll
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@ -55,6 +55,11 @@
; Slow chips use 4-byte stores. Fast chips with SSE or later use something other than 4-byte stores.
; Chips that don't have SSE use 4-byte stores either way, so they're not tested.
; Also verify that SSE4.2 or SSE4a imply fast unaligned accesses.
; RUN: llc < %s -mtriple=i386-unknown-unknown -mattr=sse4.2 2>&1 | FileCheck %s --check-prefix=FAST
; RUN: llc < %s -mtriple=i386-unknown-unknown -mattr=sse4a 2>&1 | FileCheck %s --check-prefix=FAST
define void @store_zeros(i8* %a) {
; SLOW-NOT: not a recognized processor
; SLOW-LABEL: store_zeros: