[llvm-commits] [llvm] r95809 - /llvm/trunk/lib/Target/X86/README.txt

[Eli Friedman]

Author: efriedma
Date: Wed Feb 10 15:26:04 2010
New Revision: 95809

URL: http://llvm.org/viewvc/llvm-project?rev=95809&view=rev
Log:
A few missed optimizations; the last one could have a significant impact on
code with lots of bitfields.


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=95809&r1=95808&r2=95809&view=diff

==============================================================================
--- llvm/trunk/lib/Target/X86/README.txt (original)
+++ llvm/trunk/lib/Target/X86/README.txt Wed Feb 10 15:26:04 2010
@@ -1868,3 +1868,69 @@
 information to add the "lock" prefix.
 
 //===---------------------------------------------------------------------===//
+
+_Bool bar(int *x) { return *x & 1; }
+
+define zeroext i1 @bar(i32* nocapture %x) nounwind readonly {
+entry:
+  %tmp1 = load i32* %x                            ; <i32> [#uses=1]
+  %and = and i32 %tmp1, 1                         ; <i32> [#uses=1]
+  %tobool = icmp ne i32 %and, 0                   ; <i1> [#uses=1]
+  ret i1 %tobool
+}
+
+bar:                                                        # @bar
+# BB#0:                                                     # %entry
+	movl	4(%esp), %eax
+	movb	(%eax), %al
+	andb	$1, %al
+	movzbl	%al, %eax
+	ret
+
+Missed optimization: should be movl+andl.
+
+//===---------------------------------------------------------------------===//
+
+Consider the following two functions compiled with clang:
+_Bool foo(int *x) { return !(*x & 4); }
+unsigned bar(int *x) { return !(*x & 4); }
+
+foo:
+	movl	4(%esp), %eax
+	testb	$4, (%eax)
+	sete	%al
+	movzbl	%al, %eax
+	ret
+
+bar:
+	movl	4(%esp), %eax
+	movl	(%eax), %eax
+	shrl	$2, %eax
+	andl	$1, %eax
+	xorl	$1, %eax
+	ret
+
+The second function generates more code even though the two functions are
+are functionally identical.
+
+//===---------------------------------------------------------------------===//
+
+Take the following C code:
+int x(int y) { return (y & 63) << 14; }
+
+Code produced by gcc:
+	andl	$63, %edi
+	sall	$14, %edi
+	movl	%edi, %eax
+	ret
+
+Code produced by clang:
+	shll	$14, %edi
+	movl	%edi, %eax
+	andl	$1032192, %eax
+	ret
+
+The code produced by gcc is 3 bytes shorter.  This sort of construct often
+shows up with bitfields.
+
+//===---------------------------------------------------------------------===//