[llvm-commits] [llvm] r60807 - in /llvm/trunk: lib/Target/X86/X86FastISel.cpp test/CodeGen/X86/add-with-overflow.ll

[Evan Cheng]

On Dec 10, 2008, at 11:10 AM, Bill Wendling wrote:

> On Wed, Dec 10, 2008 at 9:16 AM, Evan Cheng <echeng at apple.com> wrote:
>>
>>> +    const MachineInstr *SetMI = 0;
>>> +    unsigned Reg = lookUpRegForValue(EI);
>>> +
>>> +    for (MachineBasicBlock::const_reverse_iterator
>>> +           RI = MBB->rbegin(), RE = MBB->rend(); RI != RE; ++RI) {
>>> +      const MachineInstr &MI = *RI;
>>
>> Hi Bill,
>>
>> I don't think it makes sense to traverse machine instructions here.  
>> We
>> want fastisel to be fast and this has the potential to be slow. This
>> code is only correct when we are doing top down fastisel, right? If
>> that's the case why not just check the end of MBB instruction is a
>> SETO / SETC? If there are other instructions in between, then we'll  
>> do
>> dag isel instead. This allows the code to handle the most common  
>> cases.
>>
> The last instruction at this point isn't a SETO/SETC instruction, but
> a (useless) move of the register that SETO/SETC defines into another
> register. That's why I check whether it's a move instruction. The code
> looks something like this:
>
>        %reg1024<def> = MOV32rm <fi#-1>, 1, %reg0, 0, Mem:LD(4,4)
> [FixedStack-1 + 0]
>        %reg1025<def> = MOV32rm <fi#-2>, 1, %reg0, 0, Mem:LD(4,4)
> [FixedStack-2 + 0]
>        %reg1028<def> = ADD32rr %reg1024, %reg1025, %EFLAGS<imp-def>
>        %reg1029<def> = SETOr %EFLAGS<imp-use>
>        %reg1026<def> = MOV32rr %reg1028
>        %reg1027<def> = MOV8rr %reg1029
>
> with the conditional for the BRCOND being %reg1027. If I only look at
> the last instruction, then this will never trigger. I *could* look at
> the last three instructions, but that's pretty restrictive . . . but
> fast.

Where are the moves coming from?

Evan

>
>
> Comments?
>
>> Also, it should first check if the source of extractvalue comes from
>> arithmetic with overflow intrinsics. If not, then the code should not
>> be looking for SETO / SETC at all.
>>
> Okay.
>
> -bw
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