On Wed, Jun 24, 2015 at 12:33 AM, Philip Guenther <[email protected]> wrote: > On Tue, Jun 23, 2015 at 1:18 PM, Mike Larkin <[email protected]> wrote: >> On Tue, Jun 23, 2015 at 09:49:26AM +0300, Alexey Dobriyan wrote: >>> Clearing 32-bit register clears whole register, save REX prefix. >> >> I see nothing documented in the Intel SDM that says this. Can you cite >> a reference to support this claim? > > It's true. Volume 1, section 3.4.1 says this:
... > 9.2.1 Use Legacy 32-Bit Instructions When Data Size Is 32 Bits XOR r32, r32 is also documented to break register dependencies while XOR r64, r64 is not (15.3.2.5 zeroing idioms). Just noticed, subject line is bogus, it should be "xorq => xorl" obviously.
