Author: lattner
Date: Mon Feb 18 12:30:13 2008
New Revision: 47278
URL: http://llvm.org/viewvc/llvm-project?rev=47278&view=rev
Log:
Add a note about sext from i1 plus flags use.
Modified:
llvm/trunk/lib/Target/X86/README.txt
Modified: llvm/trunk/lib/Target/X86/README.txt
URL:
http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/X86/README.txt?rev=47278&r1=47277&r2=47278&view=diff
==============================================================================
--- llvm/trunk/lib/Target/X86/README.txt (original)
+++ llvm/trunk/lib/Target/X86/README.txt Mon Feb 18 12:30:13 2008
@@ -1528,3 +1528,55 @@
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
+
+//===---------------------------------------------------------------------===//
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