another xform that is target-independent (should be done in instcombine).

llvm-svn: 73472

llvm/lib/Target/X86/README.txt

@@ -1895,20 +1895,40 @@ Ideal output:
 
 Testcase:
 int x(int a) { return (a & 0x80) ? 0x100 : 0; }
+int y(int a) { return (a & 0x80) *2; }
 
-Current output:
+Current:
 	testl	$128, 4(%esp)
 	setne	%al
 	movzbl	%al, %eax
 	shll	$8, %eax
 	ret
 
-Ideal output:
+Better:
 	movl	4(%esp), %eax
 	addl	%eax, %eax
 	andl	$256, %eax
 	ret
 
-We generally want to fold shifted tests of a single bit into a shift+and on x86.
+This is another general instcombine transformation that is profitable on all
+targets.  In LLVM IR, these functions look like this:
+
+define i32 @x(i32 %a) nounwind readnone {
+entry:
+	%0 = and i32 %a, 128
+	%1 = icmp eq i32 %0, 0
+	%iftmp.0.0 = select i1 %1, i32 0, i32 256
+	ret i32 %iftmp.0.0
+}
+
+define i32 @y(i32 %a) nounwind readnone {
+entry:
+	%0 = shl i32 %a, 1
+	%1 = and i32 %0, 256
+	ret i32 %1
+}
+
+Replacing an icmp+select with a shift should always be considered profitable in
+instcombine.
 
 //===---------------------------------------------------------------------===//