[PATCH] D12288: make fast unaligned memory accesses implicit with SSE4.2 or SSE4a

[Sanjay Patel via llvm-commits]

spatel created this revision.
spatel added reviewers: zansari, chandlerc, qcolombet, RKSimon, silvas.
spatel added a subscriber: llvm-commits.

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

Currently, we use different store types depending on whether the example can be lowered as a memset or not.

http://reviews.llvm.org/D12288

Files:
  lib/Target/X86/X86Subtarget.cpp
  test/CodeGen/X86/slow-unaligned-mem.ll

Index: test/CodeGen/X86/slow-unaligned-mem.ll
===================================================================
--- test/CodeGen/X86/slow-unaligned-mem.ll
+++ test/CodeGen/X86/slow-unaligned-mem.ll
@@ -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:
Index: lib/Target/X86/X86Subtarget.cpp
===================================================================
--- lib/Target/X86/X86Subtarget.cpp
+++ lib/Target/X86/X86Subtarget.cpp
@@ -192,6 +192,13 @@
   // 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 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


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