[AMDGPU][GlobalISel] Allow bitcast of bf16

[shiltian]

This is a prime patch for #80908. This change doesn't break anything, though we can't test that much, given what mentioned in https://github.com/llvm/llvm-project/pull/80908/files#r1483394728.

[llvmbot]

@llvm/pr-subscribers-llvm-globalisel

Author: Shilei Tian (shiltian)

Changes

---

Full diff: https://github.com/llvm/llvm-project/pull/81674.diff

1 Files Affected:

• (modified) llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp (+3-2)

diff --git a/llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp b/llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp
index 311dd9d9739a6d..3290262816ef0a 100644
--- a/llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp
@@ -1562,8 +1562,9 @@ bool IRTranslator::translateBitCast(const User &U,
 
 bool IRTranslator::translateCast(unsigned Opcode, const User &U,
                                  MachineIRBuilder &MIRBuilder) {
-  if (U.getType()->getScalarType()->isBFloatTy() ||
-      U.getOperand(0)->getType()->getScalarType()->isBFloatTy())
+  if (Opcode != TargetOpcode::G_BITCAST &&
+      (U.getType()->getScalarType()->isBFloatTy() ||
+       U.getOperand(0)->getType()->getScalarType()->isBFloatTy()))
     return false;
   Register Op = getOrCreateVReg(*U.getOperand(0));
   Register Res = getOrCreateVReg(U);

[rampitec]

It is there for a reason. You cannot really enable it without implementing global isel support for bf16. It will start failing silently. The point of this code to fail with an error. It will also start silently failing for multiple targets.

[shiltian]

It is there for a reason. You cannot really enable it without implementing global isel support for bf16. It will start failing silently. The point of this code to fail with an error. It will also start silently failing for multiple targets.

Okay. Then I'll disable the test in #80908 since it is not supported yet.

[arsenm]

It is there for a reason. You cannot really enable it without implementing global isel support for bf16. It will start failing silently. The point of this code to fail with an error. It will also start silently failing for multiple targets.

You can, bitcast doesn't count as a bfloat operation. As long as we fail on all the actual bfloat operations, it's OK.