[llvm] [SPIR-V] Fix OpDecorate insertion point for non-PHI defs in decorateUsesAsNonUniform (PR #212524)

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<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-spir-v

Author: Arseniy Obolenskiy (aobolensk)

<details>
<summary>Changes</summary>

Inserting before the defining instruction placed the decoration ahead of its own operand, breaking dominance for the loop-carried case

Fix machine code errors from https://github.com/llvm/llvm-project/pull/208224#issuecomment-5095074660

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Full diff: https://github.com/llvm/llvm-project/pull/212524.diff


2 Files Affected:

- (modified) llvm/lib/Target/SPIRV/SPIRVInstructionSelector.cpp (+2-1) 
- (modified) llvm/test/CodeGen/SPIRV/hlsl-resources/NonUniformIdx/RWBufferNonUniformIdxLoop.ll (+1-1) 


``````````diff
diff --git a/llvm/lib/Target/SPIRV/SPIRVInstructionSelector.cpp b/llvm/lib/Target/SPIRV/SPIRVInstructionSelector.cpp
index c8f705e0251b6..a45a978b5d11e 100644
--- a/llvm/lib/Target/SPIRV/SPIRVInstructionSelector.cpp
+++ b/llvm/lib/Target/SPIRV/SPIRVInstructionSelector.cpp
@@ -6452,7 +6452,8 @@ void SPIRVInstructionSelector::decorateUsesAsNonUniform(
 
     if (!IsDecorated) {
       MachineBasicBlock &MBB = *DefMI->getParent();
-      MachineInstr &InsertPt = DefMI->isPHI() ? *MBB.getFirstNonPHI() : *DefMI;
+      MachineInstr &InsertPt =
+          DefMI->isPHI() ? *MBB.getFirstNonPHI() : *DefMI->getNextNode();
       buildOpDecorate(CurrentReg, InsertPt, TII,
                       SPIRV::Decoration::NonUniformEXT, {});
     }
diff --git a/llvm/test/CodeGen/SPIRV/hlsl-resources/NonUniformIdx/RWBufferNonUniformIdxLoop.ll b/llvm/test/CodeGen/SPIRV/hlsl-resources/NonUniformIdx/RWBufferNonUniformIdxLoop.ll
index e9dda9e94515c..ca892fefd3af4 100644
--- a/llvm/test/CodeGen/SPIRV/hlsl-resources/NonUniformIdx/RWBufferNonUniformIdxLoop.ll
+++ b/llvm/test/CodeGen/SPIRV/hlsl-resources/NonUniformIdx/RWBufferNonUniformIdxLoop.ll
@@ -1,4 +1,4 @@
-; RUN: llc -O0 -mtriple=spirv1.6-unknown-vulkan1.3-compute %s -o - | FileCheck %s
+; RUN: llc -O0 -mtriple=spirv1.6-unknown-vulkan1.3-compute -verify-machineinstrs %s -o - | FileCheck %s
 ; RUN: %if spirv-tools %{ llc -O0 -mtriple=spirv1.6-unknown-vulkan1.3-compute %s -o - -filetype=obj | spirv-val %}
 
 ; A non-uniform index carried across a loop through a phi creates an SSA cycle

``````````

</details>


https://github.com/llvm/llvm-project/pull/212524