[llvm-commits] [llvm] r135996 - /llvm/trunk/lib/Target/X86/README.txt
Benjamin Kramer
benny.kra at googlemail.com
Mon Jul 25 15:30:00 PDT 2011
Author: d0k
Date: Mon Jul 25 17:30:00 2011
New Revision: 135996
URL: http://llvm.org/viewvc/llvm-project?rev=135996&view=rev
Log:
Add a note about efficient codegen for binary log.
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=135996&r1=135995&r2=135996&view=diff
==============================================================================
--- llvm/trunk/lib/Target/X86/README.txt (original)
+++ llvm/trunk/lib/Target/X86/README.txt Mon Jul 25 17:30:00 2011
@@ -2066,3 +2066,51 @@
//===---------------------------------------------------------------------===//
+unsigned log2(unsigned x) {
+ return x > 1 ? 32-__builtin_clz(x-1) : 0;
+}
+
+generates (x86_64):
+ xorl %eax, %eax
+ cmpl $2, %edi
+ jb LBB0_2
+## BB#1:
+ decl %edi
+ movl $63, %eax
+ bsrl %edi, %ecx
+ cmovel %eax, %ecx
+ xorl $31, %ecx
+ movl $32, %eax
+ subl %ecx, %eax
+LBB0_2:
+ ret
+
+The cmov and the early test are redundant:
+ xorl %eax, %eax
+ cmpl $2, %edi
+ jb LBB0_2
+## BB#1:
+ decl %edi
+ bsrl %edi, %ecx
+ xorl $31, %ecx
+ movl $32, %eax
+ subl %ecx, %eax
+LBB0_2:
+ ret
+
+If we want to get really fancy we could use some two's complement magic:
+ xorl %eax, %eax
+ cmpl $2, %edi
+ jb LBB0_2
+## BB#1:
+ decl %edi
+ bsrl %edi, %ecx
+ xorl $-32, %ecx
+ leal 33(%ecx), %eax
+LBB0_2:
+ ret
+
+This is only useful on targets that can't encode the first operand of a sub
+directly. The rule is C1 - (X^C2) -> (C1+1) + (X^~C2).
+
+//===---------------------------------------------------------------------===//
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