[X86] Avoid emitting unnecessary test instructions.

This patch teaches the backend how to check for the 'NoSignedWrap' flag on
binary operations to improve the emission of 'test' instructions.

If the result of a binary operation is known not to overflow we know that
resetting the Overflow flag is unnecessary and so we can avoid emitting
the test instruction.

Patch by Marcello Maggioni.

llvm-svn: 210468

1 files changed, 24 insertions, 0 deletions

diff --git a/llvm/test/CodeGen/X86/2014-05-29-factorial.ll b/llvm/test/CodeGen/X86/2014-05-29-factorial.ll
new file mode 100644
index 00000000000..70255bd3219
--- /dev/null
+++ b/llvm/test/CodeGen/X86/2014-05-29-factorial.ll
@@ -0,0 +1,24 @@
+; RUN: llc < %s -march=x86-64 | FileCheck %s
+; CHECK: decq %rdi
+; CHECK-NOT: testq %rdi, %rdi
+
+define i64 @fact2(i64 %x) {
+entry:
+  br label %while.body
+
+while.body:
+  %result.06 = phi i64 [ %mul, %while.body ], [ 1, %entry ]
+  %x.addr.05 = phi i64 [ %dec, %while.body ], [ %x, %entry ]
+  %mul = mul nsw i64 %result.06, %x.addr.05
+  %dec = add nsw i64 %x.addr.05, -1
+  %cmp = icmp sgt i64 %dec, 0
+  br i1 %cmp, label %while.body, label %while.end.loopexit
+
+while.end.loopexit:
+  %mul.lcssa = phi i64 [ %mul, %while.body ]
+  br label %while.end
+
+while.end:
+  %result.0.lcssa = phi i64 [ %mul.lcssa, %while.end.loopexit ]
+  ret i64 %result.0.lcssa
+}