Add a note about sext from i1 plus flags use.

llvm-svn: 47278

lib/Target/X86/README.txt

@@ -1528,3 +1528,55 @@ int x(int a, int b) {
 See PR2053 for more details.
 
 //===---------------------------------------------------------------------===//
+
+Consider:
+int test(unsigned long a, unsigned long b) { return -(a < b); }
+
+We currently compile this to:
+
+define i32 @test(i32 %a, i32 %b) nounwind  {
+	%tmp3 = icmp ult i32 %a, %b		; <i1> [#uses=1]
+	%tmp34 = zext i1 %tmp3 to i32		; <i32> [#uses=1]
+	%tmp5 = sub i32 0, %tmp34		; <i32> [#uses=1]
+	ret i32 %tmp5
+}
+
+and
+
+_test:
+	movl	8(%esp), %eax
+	cmpl	%eax, 4(%esp)
+	setb	%al
+	movzbl	%al, %eax
+	negl	%eax
+	ret
+
+Several deficiencies here.  First, we should instcombine zext+neg into sext:
+
+define i32 @test2(i32 %a, i32 %b) nounwind  {
+	%tmp3 = icmp ult i32 %a, %b		; <i1> [#uses=1]
+	%tmp34 = sext i1 %tmp3 to i32		; <i32> [#uses=1]
+	ret i32 %tmp34
+}
+
+However, before we can do that, we have to fix the bad codegen that we get for
+sext from bool:
+
+_test2:
+	movl	8(%esp), %eax
+	cmpl	%eax, 4(%esp)
+	setb	%al
+	movzbl	%al, %eax
+	shll	$31, %eax
+	sarl	$31, %eax
+	ret
+
+This code should be at least as good as the code above.  Once this is fixed, we
+can optimize this specific case even more to:
+
+	movl	8(%esp), %eax
+	xorl	%ecx, %ecx
+        cmpl    %eax, 4(%esp)
+        sbbl    %ecx, %ecx
+
+//===---------------------------------------------------------------------===//