[AMDGPU][NPM] Preserve analyses in AMDGPURewriteAGPRCopyMFMA for NPM

[PrasoonMishra]

The pass preserved LiveStacksAnalysis but failed to preserve LiveIntervalsAnalysis, LiveRegMatrixAnalysis, VirtRegMapAnalysis, and SlotIndexesAnalysis under NPM. This caused these analyses to be invalidated and recomputed, leading to incorrect behavior in subsequent passes like VirtRegRewriter.

Fix by explicitly preserving all required analyses in the NPM version, matching the legacy pass manager behavior.

[llvmbot]

@llvm/pr-subscribers-backend-amdgpu

Author: Prasoon Mishra (PrasoonMishra)

Changes

The pass preserved LiveStacksAnalysis but failed to preserve LiveIntervalsAnalysis, LiveRegMatrixAnalysis, VirtRegMapAnalysis, and SlotIndexesAnalysis under NPM. This caused these analyses to be invalidated and recomputed, leading to incorrect behavior in subsequent passes like VirtRegRewriter.

Fix by explicitly preserving all required analyses in the NPM version, matching the legacy pass manager behavior.

---

Full diff: https://github.com/llvm/llvm-project/pull/170130.diff

1 Files Affected:

• (modified) llvm/lib/Target/AMDGPU/AMDGPURewriteAGPRCopyMFMA.cpp (+7-2)

diff --git a/llvm/lib/Target/AMDGPU/AMDGPURewriteAGPRCopyMFMA.cpp b/llvm/lib/Target/AMDGPU/AMDGPURewriteAGPRCopyMFMA.cpp
index 89c16dadb4b41..095f23ad22d97 100644
--- a/llvm/lib/Target/AMDGPU/AMDGPURewriteAGPRCopyMFMA.cpp
+++ b/llvm/lib/Target/AMDGPU/AMDGPURewriteAGPRCopyMFMA.cpp
@@ -32,6 +32,7 @@
 #include "llvm/CodeGen/LiveStacks.h"
 #include "llvm/CodeGen/MachineFrameInfo.h"
 #include "llvm/CodeGen/MachineFunctionPass.h"
+#include "llvm/CodeGen/SlotIndexes.h"
 #include "llvm/CodeGen/VirtRegMap.h"
 #include "llvm/InitializePasses.h"
 
@@ -659,7 +660,11 @@ AMDGPURewriteAGPRCopyMFMAPass::run(MachineFunction &MF,
   if (!Impl.run(MF))
     return PreservedAnalyses::all();
   auto PA = getMachineFunctionPassPreservedAnalyses();
-  PA.preserveSet<CFGAnalyses>();
-  PA.preserve<LiveStacksAnalysis>();
+  PA.preserveSet<CFGAnalyses>()
+      .preserve<LiveStacksAnalysis>()
+      .preserve<VirtRegMapAnalysis>()
+      .preserve<SlotIndexesAnalysis>()
+      .preserve<LiveIntervalsAnalysis>()
+      .preserve<LiveRegMatrixAnalysis>();
   return PA;
 }

[PrasoonMishra]

Without this patch, the following lit test fail when NPM is enabled:

LLVM :: CodeGen/AMDGPU/rewrite-vgpr-mfma-to-agpr-phi.ll
LLVM :: CodeGen/AMDGPU/rewrite-vgpr-mfma-to-agpr.gfx90a.ll
LLVM :: CodeGen/AMDGPU/rewrite-vgpr-mfma-to-agpr.gfx950.ll
LLVM :: CodeGen/AMDGPU/rewrite-vgpr-mfma-to-agpr.ll
LLVM :: CodeGen/AMDGPU/unspill-vgpr-after-rewrite-vgpr-mfma.ll
LLVM :: CodeGen/AMDGPU/inflate-reg-class-vgpr-mfma-to-av-with-load-source.mir

This patch aligns the NPM behavior with the legacy pass manager (see AMDGPURewriteAGPRCopyMFMA.cpp Line 607-610).

[cdevadas]

I guess SlotIndex is not directly used here, but invoked inside LiveIntervals?
It doesn't seem good to me to explicitly preserve the analyses that aren't explicitly used in a pass. Rather, the pass manager should have some way to preserve them. In this case LiveIntervals analysis is preserved here. The pass-manager infrastructure should preserve the SlitIndex too. Anyway, it doesn't work that way at the moment.

[PrasoonMishra]

Thanks for pointing that out. I agree the current approach isn’t ideal, but given the limitations of the pass manager right now, I’ll leave it as is.