[llvm] [RISCV][GISEL] Fix legalization for G_MERGE/UNMERGE_VALUES (PR #75619)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

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

Author: Michael Maitland (michaelmaitland)

<details>
<summary>Changes</summary>

There are two bugs fixed by this patch:

  1. G_MERGE_VALUES is always true, causing IdxZeroTy and IdxOneTy to always be s64 and s32 respectively.
  2. The legalFor check was for the type in index 0, instead of for index 0 and index 1, so we were allowing s64 or s32 in type 0 to be legal for both G_MERGE_VALUES and G_UNMERGE_VALUES.

The existing test was passing because (2) covered up (1). 

---
Full diff: https://github.com/llvm/llvm-project/pull/75619.diff


1 Files Affected:

- (modified) llvm/lib/Target/RISCV/GISel/RISCVLegalizerInfo.cpp (+7-5) 


``````````diff
diff --git a/llvm/lib/Target/RISCV/GISel/RISCVLegalizerInfo.cpp b/llvm/lib/Target/RISCV/GISel/RISCVLegalizerInfo.cpp
index 8f03a7ac41d37b..b46269fb9565fb 100644
--- a/llvm/lib/Target/RISCV/GISel/RISCVLegalizerInfo.cpp
+++ b/llvm/lib/Target/RISCV/GISel/RISCVLegalizerInfo.cpp
@@ -85,14 +85,16 @@ RISCVLegalizerInfo::RISCVLegalizerInfo(const RISCVSubtarget &ST)
 
   // Merge/Unmerge
   for (unsigned Op : {G_MERGE_VALUES, G_UNMERGE_VALUES}) {
-    unsigned BigTyIdx = Op == G_MERGE_VALUES ? 0 : 1;
-    unsigned LitTyIdx = Op == G_MERGE_VALUES ? 1 : 0;
     auto &MergeUnmergeActions = getActionDefinitionsBuilder(Op);
     if (XLen == 32 && ST.hasStdExtD()) {
-      LLT IdxZeroTy = G_MERGE_VALUES ? s64 : s32;
-      LLT IdxOneTy = G_MERGE_VALUES ? s32 : s64;
-      MergeUnmergeActions.legalFor({IdxZeroTy, IdxOneTy});
+      // Only need to legalize on IdxZeroTy since there is no way to construct
+      // a valid MERGE/UNMERGE where the types don't combine/decompose as
+      // expected.
+      LLT IdxZeroTy = Op == G_MERGE_VALUES ? s64 : s32;
+      MergeUnmergeActions.legalFor({IdxZeroTy});
     }
+    unsigned BigTyIdx = Op == G_MERGE_VALUES ? 0 : 1;
+    unsigned LitTyIdx = Op == G_MERGE_VALUES ? 1 : 0;
     MergeUnmergeActions.widenScalarToNextPow2(LitTyIdx, XLen)
         .widenScalarToNextPow2(BigTyIdx, XLen)
         .clampScalar(LitTyIdx, sXLen, sXLen)

``````````

</details>


https://github.com/llvm/llvm-project/pull/75619