Add a note about efficient codegen for binary log.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@135996 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Target/X86/README.txt b/lib/Target/X86/README.txt
index 560947a4a04b79882b77d5994a0eab6842eed9c0..d6ceaa7ed4b614ba60f606cb22aa1e7e4abfe71a 100644 (file)
--- a/lib/Target/X86/README.txt
+++ b/lib/Target/X86/README.txt
@@ -2066,3 +2066,51 @@ The trick is to match "fetch_and_add(X, -C) == C".
 
 //===---------------------------------------------------------------------===//
 
+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).
+
+//===---------------------------------------------------------------------===//