[GlobalISel][KnownBits] Simplify G_CONSTANT handling

[Pierre-vh]

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.

[llvmbot]

@llvm/pr-subscribers-llvm-globalisel

Author: Pierre van Houtryve (Pierre-vh)

Changes

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 --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: {