[PATCH] D127996: [RISCV] Delete dead elideCopy code in InsertVSETVLI [nfc]

[Craig Topper via Phabricator via llvm-commits]

craig.topper added a comment.

In D127996#3590034 <https://reviews.llvm.org/D127996#3590034>, @craig.topper wrote:

> Looking at this more, this code may only be dead because we don't do the tied operand check if a policy operand is present. That's probably true for a significant number of instructions these days.
>
> Ignoring that and looking at what the MIR looks like at the time we reach InsertVSETVLI for this test case
>
>   define <vscale x 8 x i64> @vpload_nxv8i64(<vscale x 8 x i64>* %ptr, <vscale x 8 x i1> %m, i32 zeroext %evl) #1 {
>     %load = call <vscale x 8 x i64> @llvm.vp.load.nxv8i64.p0nxv8i64(<vscale x 8 x i64>* %ptr, <vscale x 8 x i1> %m, i32 %evl)
>     ret <vscale x 8 x i64> %load                                                   
>   } 
>
> I see
>
>   # Machine code for function vpload_nxv8i64: IsSSA, TracksLiveness                
>   Function Live Ins: $x10 in %0, $v0 in %1, $x11 in %2                             
>                                                                                    
>   bb.0 (%ir-block.0):                                                              
>     liveins: $x10, $v0, $x11                                                       
>     %2:gprnox0 = COPY $x11                                                         
>     %1:vr = COPY $v0                                                               
>     %0:gpr = COPY $x10                                                             
>     $v0 = COPY %1:vr                                                               
>     %4:vrm8 = IMPLICIT_DEF                                                         
>     %5:vrm8nov0 = COPY %4:vrm8                                                     
>     %3:vrm8nov0 = PseudoVLE64_V_M8_MASK %5:vrm8nov0(tied-def 0), %0:gpr, $v0, %2:gprnox0, 6, 1 :: (load unknown-size from %ir.ptr, align 64)
>     $v8m8 = COPY %3:vrm8nov0                                                       
>     PseudoRET implicit $v8m8                                                       
>                                                                                    
>   # End machine code for function vpload_nxv8i64. 
>
> That shows an IMPLICIT_DEF behind a copy instruction. I think it is related to vrm8nov0 having 3 registers. It doesn't happen on an LMUL=4 test. I suspect it hit the MinRCSize limit of 4 in InstrEmitter::AddRegisterOperand.

I think this might be a bug in InstrEmitter. InstrEmitter::getVR creates unique vreg for every IMPLICIT_DEF use, but then we aren't allowed to constrain it below MinRCSize.


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  https://reviews.llvm.org/D127996/new/

https://reviews.llvm.org/D127996