implement rdar://6653118 - fastisel should fold loads where possible.

Since mem2reg isn't run at -O0, we get a ton of reloads from the stack,
for example, before, this code:

int foo(int x, int y, int z) {
  return x+y+z;
}

used to compile into:

_foo:                                   ## @foo
	subq	$12, %rsp
	movl	%edi, 8(%rsp)
	movl	%esi, 4(%rsp)
	movl	%edx, (%rsp)
	movl	8(%rsp), %edx
	movl	4(%rsp), %esi
	addl	%edx, %esi
	movl	(%rsp), %edx
	addl	%esi, %edx
	movl	%edx, %eax
	addq	$12, %rsp
	ret

Now we produce:

_foo:                                   ## @foo
	subq	$12, %rsp
	movl	%edi, 8(%rsp)
	movl	%esi, 4(%rsp)
	movl	%edx, (%rsp)
	movl	8(%rsp), %edx
	addl	4(%rsp), %edx    ## Folded load
	addl	(%rsp), %edx     ## Folded load
	movl	%edx, %eax
	addq	$12, %rsp
	ret

Fewer instructions and less register use = faster compiles.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@113102 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 14 insertions, 4 deletions

diff --git a/test/CodeGen/X86/fast-isel-mem.ll b/test/CodeGen/X86/fast-isel-mem.ll
index 35ec1e7..8db1936 100644
--- a/test/CodeGen/X86/fast-isel-mem.ll
+++ b/test/CodeGen/X86/fast-isel-mem.ll
@@ -1,10 +1,8 @@
-; RUN: llc < %s -fast-isel -mtriple=i386-apple-darwin | \
-; RUN:   grep lazy_ptr, | count 2
-; RUN: llc < %s -fast-isel -march=x86 -relocation-model=static | \
-; RUN:   grep lea
+; RUN: llc < %s -fast-isel -mtriple=i386-apple-darwin | FileCheck %s
 
 @src = external global i32
 
+; rdar://6653118
 define i32 @loadgv() nounwind {
 entry:
 	%0 = load i32* @src, align 4
@@ -12,6 +10,14 @@ entry:
         %2 = add i32 %0, %1
         store i32 %2, i32* @src
 	ret i32 %2
+; This should fold one of the loads into the add.
+; CHECK: loadgv:
+; CHECK: 	movl	L_src$non_lazy_ptr, %ecx
+; CHECK: 	movl	(%ecx), %eax
+; CHECK: 	addl	(%ecx), %eax
+; CHECK: 	movl	%eax, (%ecx)
+; CHECK: 	ret
+
 }
 
 %stuff = type { i32 (...)** }
@@ -21,4 +27,8 @@ define void @t(%stuff* %this) nounwind {
 entry:
 	store i32 (...)** getelementptr ([4 x i32 (...)*]* @LotsStuff, i32 0, i32 2), i32 (...)*** null, align 4
 	ret void
+; CHECK: _t:
+; CHECK:	movl	$0, %eax
+; CHECK:	movl	L_LotsStuff$non_lazy_ptr, %ecx
+
 }