[llvm] [AMDGPU] Mark PC_ADD_REL_OFFSET rematerializable (PR #79674)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-amdgpu

Author: Yaxun (Sam) Liu (yxsamliu)

<details>
<summary>Changes</summary>

Currently machine LICM hoist PC_ADD_REL_OFFSET out of loops, causes register pressure when function calls are deep in loops. This is a main cause of sgpr spill for programs containing large number of function calls in loops.

This patch marks PC_ADD_REL_OFFSET as rematerializable, which eliminates sgpr spills due to function calls in loops. It significantly improves performance of HIP apps which have large number of function calls, e.g., Blender.

---
Full diff: https://github.com/llvm/llvm-project/pull/79674.diff


1 Files Affected:

- (modified) llvm/lib/Target/AMDGPU/SIInstructions.td (+1) 


``````````diff
diff --git a/llvm/lib/Target/AMDGPU/SIInstructions.td b/llvm/lib/Target/AMDGPU/SIInstructions.td
index 788e3162fb37e2..b593b7dbfe0827 100644
--- a/llvm/lib/Target/AMDGPU/SIInstructions.td
+++ b/llvm/lib/Target/AMDGPU/SIInstructions.td
@@ -1036,6 +1036,7 @@ let isConvergent = 1 in {
   defm SI_SPILL_WWM_AV32 : SI_SPILL_VGPR <AV_32, 1>;
 }
 
+let isReMaterializable = 1, isAsCheapAsAMove = 1 in
 def SI_PC_ADD_REL_OFFSET : SPseudoInstSI <
   (outs SReg_64:$dst),
   (ins si_ga:$ptr_lo, si_ga:$ptr_hi),

``````````

</details>


https://github.com/llvm/llvm-project/pull/79674