[PATCH] D147096: AMDGPU: Created a sub-register class for the return address operand in the tail call return instruction

[Matt Arsenault via Phabricator via llvm-commits]

arsenm added inline comments.


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Comment at: llvm/lib/Target/AMDGPU/SIRegisterInfo.td:822
+// return address only.
+def CCR_SGPR_64 : SIRegisterClass<"AMDGPU", SGPR_64.RegTypes, 32, (add (trunc SGPR_64, 16))> {
+  let CopyCost = SGPR_64.CopyCost;
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cfang wrote:
> sebastian-ne wrote:
> > foad wrote:
> > > Doesn't this depend on the calling convention? CSR_AMDGPU_SGPRs and CSR_AMDGPU_SI_Gfx_SGPRs are different.
> > Yeah, the amdgpu_gfx calling convention has s[32:63] as scratch registers, so picking a register out of s[0:32] as return register makes tail calls with the amdgpu_gfx calling convention impossible.
> We are seeing a tail call return address in callee saved registers, which were over-written during restore of the CSR before return. This is a correct issue. To solve this, we think it is better to restrict the tail call return address to be in the scratch registers, rather than in sreg_64 which could be a CSR. 
>    I am not sure whether the same issue exists with amdgpu_gfx calling convention. If yes, we may need to do similar thing to restrict to scratch register for tail call return address.
The same problem will exist for all calling conventions 


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https://reviews.llvm.org/D147096