[llvm] [GlobalISel][KnownBits] Simplify G_CONSTANT handling (PR #91946)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-globalisel

Author: Pierre van Houtryve (Pierre-vh)

<details>
<summary>Changes</summary>

We called getIConstantVRegVal which again queried MRI to get the VReg def. We already have the def, so just get the CImm directly. It can't fail.

---
Full diff: https://github.com/llvm/llvm-project/pull/91946.diff


1 Files Affected:

- (modified) llvm/lib/CodeGen/GlobalISel/GISelKnownBits.cpp (+1-4) 


``````````diff
diff --git a/llvm/lib/CodeGen/GlobalISel/GISelKnownBits.cpp b/llvm/lib/CodeGen/GlobalISel/GISelKnownBits.cpp
index c8199a42d15c3..32d607cfd71a7 100644
--- a/llvm/lib/CodeGen/GlobalISel/GISelKnownBits.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/GISelKnownBits.cpp
@@ -256,10 +256,7 @@ void GISelKnownBits::computeKnownBitsImpl(Register R, KnownBits &Known,
     break;
   }
   case TargetOpcode::G_CONSTANT: {
-    auto CstVal = getIConstantVRegVal(R, MRI);
-    if (!CstVal)
-      break;
-    Known = KnownBits::makeConstant(*CstVal);
+    Known = KnownBits::makeConstant(MI.getOperand(1).getCImm()->getValue());
     break;
   }
   case TargetOpcode::G_FRAME_INDEX: {

``````````

</details>


https://github.com/llvm/llvm-project/pull/91946