[llvm-commits] Patch for X86 to use subregs

[Christopher Lamb]

On Jul 28, 2007, at 2:26 PM, Evan Cheng wrote:

> On Jul 28, 2007, at 11:52 AM, Christopher Lamb  
> <christopher.lamb at gmail.com> wrote:
>
>>
>> On Jul 28, 2007, at 1:48 AM, Evan Cheng wrote:
>>
>>> Very cool! I need to read it more carefully.
>>
>>> But I see you are lowering zext to a single insert_subreg. Is  
>>> that right? It won't zero out the top part, no?
>>
>> It's only lowering (zext i32 to i64) to an insert_subreg on x86-64  
>> where all writes to 32-bit registers implicitly zero-extend into  
>> the upper 32-bits.
>>
>
> I know. But thy mismatch semantically. A insert_subreg to the lower  
> part should not change the upper half. I think this is only legal  
> for anyext.

On x86-64 the semantics of a 2 operand i32 insert_subreg is that the  
input super-value is implicitly zero. So in this sense the insert  
isn't changing the upper half, it's just that the upper half is being  
set to zero implicitly rather than explicitly. If you'll notice the  
insert_subreg is a two operand (implicit super value) not a three  
operand version. If the insert were the three operand version, and  
the super value as coming from an implicit def I'd agree with you,  
but it's not.

Also the current behavior is to use a 32-bit mov instruction for both  
zeroext and for anyext, I don't see how this is any different.

>> --
>> Chris
>>
>>> Sent from my iPhone
>>>
>>> On Jul 28, 2007, at 12:17 AM, Christopher Lamb  
>>> <christopher.lamb at gmail.com> wrote:
>>>
>>>> This patch changes the X86 back end to use the new subreg  
>>>> operations for appropriate truncate and extend operations. This  
>>>> should allow regression testing of the subreg feature going  
>>>> forward, as it's now used in a public target.
>>>>
>>>> The patch passed DejaGnu and all of SingleSource on my x86  
>>>> machine, but there are changes for x86-64 as well which I  
>>>> haven't been able to test. Output assembly for x86-64 appears  
>>>> sane, but I'd appreciate someone giving the patch a try on their  
>>>> x86-64 system. Other 32-bit x86 testing is also appreciated.
>>>>
>>>> Thanks
>>>> --
>>>> Christopher Lamb
>>>>
>>>> <x86_subregs.patch>
>>>>
>>>>
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>>
>> --
>> Christopher Lamb
>>
>>
>>
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--
Christopher Lamb



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