[llvm-commits] [llvm] r61424 - in /llvm/trunk/lib/Target/X86: X86Instr64bit.td X86InstrInfo.td

[Chris Lattner]

On Dec 24, 2008, at 7:28 PM, Eli Friedman wrote:

> On Wed, Dec 24, 2008 at 5:27 PM, Chris Lattner <sabre at nondot.org>  
> wrote:
>> Author: lattner
>> Date: Wed Dec 24 19:27:10 2008
>> New Revision: 61424
>>
>> URL: http://llvm.org/viewvc/llvm-project?rev=61424&view=rev
>> Log:
>> BT memory operands load from their address operand.
>
> Do we really want to map this to the same intrinsic as the register
> form?  "mov (%esp), %eax; bt %ebx, %eax;" has different semantics from
> "bt %ebx, (%esp), ", and the latter form is significantly slower.

It does?  How so?  The later form reduces register pressure, so absent  
a difference in semantics it is preferable.

-Chris