[PATCH] D149775: [AMDGPU] Reserve SGPR pair when long branches are present

Fri Jun 9 13:24:27 PDT 2023

crobeck added inline comments.

================
Comment at: llvm/lib/Target/AMDGPU/SIFrameLowering.cpp:1453-1455
+    Register UnusedLowSGPR =
+        TRI->findUnusedRegister(MRI, &AMDGPU::SGPR_64RegClass, MF);
+    FuncInfo->setLongBranchReservedReg(UnusedLowSGPR);
----------------
crobeck wrote:
> crobeck wrote:
> > Pierre-vh wrote:
> > > How does this work? Don't you need to check if `UnusedLowSGPR` returns something? Will it always return the pair we reserved earlier if nothing else is available?
> > > 
> > > Would it be worth adding a test with `amdgpu-num-sgpr` to see what happens when 100% of SGPRs are used?
> > Yes, you're right. I've made the checks more explicit.
> > Would it be worth adding a test with `amdgpu-num-sgpr` to see what happens when 100% of SGPRs are used?
> 
> That's a good idea. At the very least good to understand what happens when were maxed out on SGPRs.
This ended up being kind of a mess trying to do it with the amdgpu-num-sgpr attribute so just used inline asm to use up all the registers

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https://reviews.llvm.org/D149775