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3 Commits

Author SHA1 Message Date
Benjamin Kramer b16bd77bd2 InstCombine: Add a combine that turns (2^n)-1 ^ x back into (2^n)-1 - x iff x is smaller than 2^n and it fuses with a following add. 
This was intended to undo the sub canonicalization in cases where it's not profitable, but it also
finds some cases on it's own.

llvm-svn: 147256
 2011-12-24 17:31:53 +00:00
Benjamin Kramer 4ee5747fdd ComputeMaskedBits: Make knownzero computation more aggressive for ctlz with undef zero. 
unsigned foo(unsigned x) { return 31 - __builtin_clz(x); }
now compiles into a single "bsrl" instruction on x86.

llvm-svn: 147255
 2011-12-24 17:31:46 +00:00
Benjamin Kramer 010337c838 InstCombine: Canonicalize (2^n)-1 - x into (2^n)-1 ^ x iff x is known to be smaller than 2^n. 
This has the obvious advantage of being commutable and is always a win on x86 because
const - x wastes a register there. On less weird architectures this may lead to
a regression because other arithmetic doesn't fuse with it anymore. I'll address that
problem in a followup.

llvm-svn: 147254
 2011-12-24 17:31:38 +00:00