Inline asm clobber registers name

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1

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?

Thanks,

Sam

2

From: "Семен Колтон via llvm-dev" <llvm-dev@lists.llvm.org>
To: "via llvm-dev" <llvm-dev@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() &&
PPC::GPRCRegClass.contains(R.first))
return std::make_pair(TRI->getMatchingSuperReg(R.first,
PPC::sub_32, &PPC::G8RCRegClass),
&PPC::G8RCRegClass);

-Hal

3

Hi Hal,

In that case I will start implementing it in core LLVM. What I want to do is add table with AsmNames to TargetGenRegisterInfo.inc similar to RegStrings table. This table could be used by getRegForInlineAsmConstraint.

I don’t want to force all back-ends to use AsmNames instead of def-names. There is useful review for this created by Tom Stellard: http://reviews.llvm.org/D15614.

What do you think about all this?

Thanks,

Sam

4

From: "Семен Колтон" <samkolton@gmail.com>
To: "Hal Finkel" <hfinkel@anl.gov>
Cc: "via llvm-dev" <llvm-dev@lists.llvm.org>
Sent: Friday, April 8, 2016 6:38:22 AM
Subject: Re: [llvm-dev] Inline asm clobber registers name

Hi Hal,

In that case I will start implementing it in core LLVM. What I want
to do is add table with AsmNames to TargetGenRegisterInfo.inc
similar to RegStrings table. This table could be used by
getRegForInlineAsmConstraint.
I don’t want to force all back-ends to use AsmNames instead of
def-names. There is useful review for this created by Tom Stellard:
⚙ D15614 TargetRegisterInfo: Add getRegAsmName() .
What do you think about all this?

I think this sounds reasonable. I'll look at Tom's patch.

-Hal