[llvm-commits] [llvm] r66515 - /llvm/trunk/lib/Target/X86/README.txt

[Chris Lattner]

On Mar 9, 2009, at 5:26 PM, Dan Gohman wrote:
> The shift operators have the complication that when the shift count is
> zero, EFLAGS is not set, so they can only subsume a test instruction  
> if
> +the shift count is known to be non-zero. Also, using the EFLAGS value
> +from a shift is apparently very slow on some x86 implementations.

FWIW, "flags from shift" are only slow for variable shifts (shifts by  
cl) precisely because if CL is zero the flags aren't modified.

-Chris