Teach fast-isel to avoid loading a value from memory when it's already

available in a register. This is pretty primitive, but it reduces the
number of instructions in common testcases by 4%.

llvm-svn: 107380

1 files changed, 23 insertions, 0 deletions

diff --git a/llvm/test/CodeGen/X86/fast-isel-loads.ll b/llvm/test/CodeGen/X86/fast-isel-loads.ll
new file mode 100644
index 00000000000..2fbb46c0b9f
--- /dev/null
+++ b/llvm/test/CodeGen/X86/fast-isel-loads.ll
@@ -0,0 +1,23 @@
+; RUN: llc -march=x86-64 -O0 -asm-verbose=false < %s | FileCheck %s
+
+; Fast-isel shouldn't reload the argument values from the stack.
+
+; CHECK: foo:
+; CHECK-NEXT: movq  %rdi, -8(%rsp)
+; CHECK-NEXT: movq  %rsi, -16(%rsp)
+; CHECK-NEXT: movsd 128(%rsi,%rdi,8), %xmm0
+; CHECK-NEXT: ret
+
+define double @foo(i64 %x, double* %p) nounwind {
+entry:
+  %x.addr = alloca i64, align 8                   ; <i64*> [#uses=2]
+  %p.addr = alloca double*, align 8               ; <double**> [#uses=2]
+  store i64 %x, i64* %x.addr
+  store double* %p, double** %p.addr
+  %tmp = load i64* %x.addr                        ; <i64> [#uses=1]
+  %tmp1 = load double** %p.addr                   ; <double*> [#uses=1]
+  %add = add nsw i64 %tmp, 16                     ; <i64> [#uses=1]
+  %arrayidx = getelementptr inbounds double* %tmp1, i64 %add ; <double*> [#uses=1]
+  %tmp2 = load double* %arrayidx                  ; <double> [#uses=1]
+  ret double %tmp2
+}