[llvm-commits] [llvm] r164926 - /llvm/trunk/lib/Target/X86/MCTargetDesc/X86MCTargetDesc.cpp

[Chandler Carruth]

Adding Hans, who worked on the lookup table formation...


On Mon, Oct 1, 2012 at 12:50 AM, Craig Topper <craig.topper at gmail.com>wrote:

> It was forming a jump table instead of just a constant lookup table
> because of the subtract in there making it so that not everything was a
> constant. Maybe there should be some way to detect that the subtract was
> only dependent on the switch value?
>
> Even the jump table switch had problems because it seems nothing combines
> identical basic blocks except for some common tail merging that occurs in
> branch folding. But the tail folding has a threshold of 3 instructions in
> the tail folding and these duplicate basic blocks were all 2 instructions
> long. Maybe tail merging should ignore the threshold if the common tail of
> two blocks are the whole blocks and they aren't both laid out behind their
> predecessor?
>
>
> On Mon, Oct 1, 2012 at 12:40 AM, Chandler Carruth <chandlerc at google.com>wrote:
>
>> On Mon, Oct 1, 2012 at 12:33 AM, Craig Topper <craig.topper at gmail.com>wrote:
>>
>>> Author: ctopper
>>> Date: Mon Oct  1 02:33:27 2012
>>> New Revision: 164926
>>>
>>> URL: http://llvm.org/viewvc/llvm-project?rev=164926&view=rev
>>> Log:
>>> Use constants for all return values in switch. Allows clang to optimize
>>> it into a lookup table.
>>>
>>> Modified:
>>>     llvm/trunk/lib/Target/X86/MCTargetDesc/X86MCTargetDesc.cpp
>>>
>>> Modified: llvm/trunk/lib/Target/X86/MCTargetDesc/X86MCTargetDesc.cpp
>>> URL:
>>> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/X86/MCTargetDesc/X86MCTargetDesc.cpp?rev=164926&r1=164925&r2=164926&view=diff
>>>
>>> ==============================================================================
>>> --- llvm/trunk/lib/Target/X86/MCTargetDesc/X86MCTargetDesc.cpp (original)
>>> +++ llvm/trunk/lib/Target/X86/MCTargetDesc/X86MCTargetDesc.cpp Mon Oct
>>>  1 02:33:27 2012
>>> @@ -243,9 +243,14 @@
>>>    case X86::R15: case X86::R15D: case X86::R15W: case X86::R15B:
>>>      return N86::EDI;
>>>
>>> -  case X86::ST0: case X86::ST1: case X86::ST2: case X86::ST3:
>>> -  case X86::ST4: case X86::ST5: case X86::ST6: case X86::ST7:
>>> -    return RegNo-X86::ST0;
>>> +  case X86::ST0: return 0;
>>> +  case X86::ST1: return 1;
>>> +  case X86::ST2: return 2;
>>> +  case X86::ST3: return 3;
>>> +  case X86::ST4: return 4;
>>> +  case X86::ST5: return 5;
>>> +  case X86::ST6: return 6;
>>> +  case X86::ST7: return 7;
>>>
>>
>> Out of curiosity, why are we unable to form the lookup table out of this?
>> It seems like a common pattern that should be directly supported.
>>
>>
>
>
> --
> ~Craig
>
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