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| -rw-r--r-- | lib/Target/X86/README.txt | 48 |
1 files changed, 48 insertions, 0 deletions
diff --git a/lib/Target/X86/README.txt b/lib/Target/X86/README.txt index 560947a4a0..d6ceaa7ed4 100644 --- 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). + +//===---------------------------------------------------------------------===// |