[llvm-dev] Inline asm clobber registers name

Hal Finkel via llvm-dev llvm-dev at lists.llvm.org
Thu Apr 7 08:46:54 PDT 2016

----- Original Message -----

> From: "Семен Колтон via llvm-dev" <llvm-dev at lists.llvm.org>
> To: "via llvm-dev" <llvm-dev at lists.llvm.org>
> Sent: Thursday, April 7, 2016 10:06:21 AM
> Subject: [llvm-dev] Inline asm clobber registers name

> Hi all,

> I am currently working on AMDGPU inline assembly and encountered
> problem with naming clobber registers in asm constraints. It looks
> like by default LLVM tries to match register specified in constraint
> to register name of register definition in .td file but not to the
> AsmName for this register.

> For example if we have register definition:
> def MYReg0 : Register<"r0", 0>;
> We want to create inline assembly and add this register to clobbers
> list. Inline assembly should look something like this:
> i32 asm "nop", "~{r0}" ()

> We used AsmName for register MYReg0 inside clobbers list. But this
> constraint fails to work because
> TargetLowering::getRegForInlineAsmConstraint() tries to match
> register definition name (“MYReg0”) not its AsmName (“r0”). So to
> make this work we should write this assembly:
> i32 asm "nop", "~{MYReg0}" ()

> I believe that this behavior is not correct. It works because in most
> back-ends register definition names and AsmNames are equal ( e.g.
> def EAX : X86Reg<"eax", ...> ) but in AMDGPU we want to have
> different def-names and AsmNames.

> This might be done by changing core LLVM code or in target-specific
> getRegForInlineAsmConstraint() method. What do you suppose to be
> better solution?
I agree. I have the following FIXME in PPCISelLowering: 

// r[0-9]+ are used, on PPC64, to refer to the corresponding 64-bit registers 
// (which we call X[0-9]+). If a 64-bit value has been requested, and a 
// 32-bit GPR has been selected, then 'upgrade' it to the 64-bit parent 
// register. 
// FIXME: If TargetLowering::getRegForInlineAsmConstraint could somehow use 
// the AsmName field from *RegisterInfo.td, then this would not be necessary. 
if (R.first && VT == MVT::i64 && Subtarget.isPPC64() && 
return std::make_pair(TRI->getMatchingSuperReg(R.first, 
PPC::sub_32, &PPC::G8RCRegClass), 


> Thanks,
> Sam
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Hal Finkel 
Assistant Computational Scientist 
Leadership Computing Facility 
Argonne National Laboratory 
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