[llvm-dev] Liveness of AL, AH and AX in x86 backend

[Krzysztof Parzyszek via llvm-dev]

Enabling subreg liveness tracking didn't do anything.  By altering the 
allocation order I managed to get the backend to use CL/CH for the 
struct, but the stores were still separate (even though storing CX would 
be correct)...

Here's another question that falls into the same category:

The function X86InstrInfo::loadRegFromStackSlot does not append any 
implicit uses/defs.  How does it know that it won't need them?  If AX 
was spilled in the middle of a live range of EAX, wouldn't restoring of 
AX need to implicitly define EAX?

We deal with such cases a lot in the Hexagon backend and it continues to 
be a major pain.  I'm trying to understand if there are better options 
for us.

-Krzysztof



On 5/24/2016 12:40 PM, Quentin Colombet wrote:
> Hi Krzysztof,
>
>> On May 24, 2016, at 8:03 AM, Krzysztof Parzyszek via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>>
>> I'm trying to see how the x86 backend deals with the relationship between AL, AH and AX, but I can't get it to generate any code that would expose an interesting scenario.
>>
>> For example, I wrote this piece:
>>
>> typedef struct {
>>  char x, y;
>> } struct_t;
>>
>> struct_t z;
>>
>> struct_t foo(char *p) {
>>  struct_t s;
>>  s.x = *p++;
>>  s.y = *p;
>>  z = s;
>>  s.x++;
>>  return s;
>> }
>>
>> But the output at -O2 is
>>
>> foo:                                    # @foo
>>        .cfi_startproc
>> # BB#0:                                 # %entry
>>        movb    (%rdi), %al
>>        movzbl  1(%rdi), %ecx
>>        movb    %al, z(%rip)
>>        movb    %cl, z+1(%rip)
>>        incb    %al
>>        shll    $8, %ecx
>>        movzbl  %al, %eax
>>        orl     %ecx, %eax
>>        retq
>>
>>
>> I was hoping it would do something along the lines of
>>
>>  movb (%rdi), %al
>>  movb 1(%rdi), %ah
>>  movh %ax, z(%rip)
>>  incb %al
>>  retq
>>
>>
>> Why is the x86 backend not getting this code?
>
> Try enabling the sub-register liveness feature. I am guessing we think we cannot use the same register for the low and high part.
> Though, I would need to see the machine instrs to be sure.
>
>>  Does it know that AH:AL = AX?
>
> Yes it does.
>
> Cheers,
> -Quentin
>>
>> -Krzysztof
>>
>>
>>
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>


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