MemCpyOpt: Turn memcpys from a constant into a memset if possible.

This allows us to compile "int cst[] = {-1, -1, -1};" into
  movl  $-1, 16(%rsp)
  movq  $-1, 8(%rsp)
instead of
  movl  _cst+8(%rip), %eax
  movl  %eax, 16(%rsp)
  movq  _cst(%rip), %rax
  movq  %rax, 8(%rsp)

llvm-svn: 122548

1 files changed, 19 insertions, 0 deletions

diff --git a/llvm/test/Transforms/MemCpyOpt/memcpy-to-memset.ll b/llvm/test/Transforms/MemCpyOpt/memcpy-to-memset.ll
new file mode 100644
index 00000000000..b18d176f003
--- /dev/null
+++ b/llvm/test/Transforms/MemCpyOpt/memcpy-to-memset.ll
@@ -0,0 +1,19 @@
+; RUN: opt -memcpyopt -S < %s | FileCheck %s
+
+@cst = internal constant [3 x i32] [i32 -1, i32 -1, i32 -1], align 4
+
+declare void @llvm.memcpy.p0i8.p0i8.i64(i8* nocapture, i8* nocapture, i64, i32, i1) nounwind
+declare void @foo(i32*) nounwind
+
+define void @test1() nounwind {
+  %arr = alloca [3 x i32], align 4
+  %arr_i8 = bitcast [3 x i32]* %arr to i8*
+  call void @llvm.memcpy.p0i8.p0i8.i64(i8* %arr_i8, i8* bitcast ([3 x i32]* @cst to i8*), i64 12, i32 4, i1 false)
+  %arraydecay = getelementptr inbounds [3 x i32]* %arr, i64 0, i64 0
+  call void @foo(i32* %arraydecay) nounwind
+  ret void
+; CHECK: @test1
+; CHECK: call void @llvm.memset
+; CHECK-NOT: call void @llvm.memcpy
+; CHECK: ret void
+}