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

Pierre-vh · web-flow · commit 4ff45eee36f0 · 2024-05-13T13:59:24.000+02:00
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.
diff --git 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: {

Original file line number	Diff line number	Diff line change
`@@ -256,10 +256,7 @@ void GISelKnownBits::computeKnownBitsImpl(Register R, KnownBits &Known,`
`256`	`256`	`break;`
`257`	`257`	`}`
`258`	`258`	`case TargetOpcode::G_CONSTANT: {`
`259`		`- auto CstVal = getIConstantVRegVal(R, MRI);`
`260`		`- if (!CstVal)`
`261`		`- break;`
`262`		`- Known = KnownBits::makeConstant(*CstVal);`
	`259`	`+ Known = KnownBits::makeConstant(MI.getOperand(1).getCImm()->getValue());`
`263`	`260`	`break;`
`264`	`261`	`}`
`265`	`262`	`case TargetOpcode::G_FRAME_INDEX: {`