[llvm] [GISel] Explicitly disable BF16 tablegen patterns. (PR #124113)

[David Green via llvm-commits]

https://github.com/davemgreen created https://github.com/llvm/llvm-project/pull/124113

We currently have an issue where bf16 patters can be used to match fp16 types, as GISel does not know about the difference between the two types. This patch explicitly disables them to make sure that they are never used.

>From 7e79a0633caa6edea5498ac461ccc1269e24de05 Mon Sep 17 00:00:00 2001
From: David Green <david.green at arm.com>
Date: Thu, 23 Jan 2025 13:34:59 +0000
Subject: [PATCH] [GISel] Explicitly disable BF16 tablegen patterns.

We currently have an issue where bf16 patters can be used to match fp16 types,
as GISel does not know about the difference between the two types. This patch
explicitly disables them to make sure that they are never used.
---
 llvm/test/CodeGen/AArch64/fptrunc.ll      |  1 +
 llvm/utils/TableGen/GlobalISelEmitter.cpp | 21 +++++++++++++++++++++
 2 files changed, 22 insertions(+)

diff --git a/llvm/test/CodeGen/AArch64/fptrunc.ll b/llvm/test/CodeGen/AArch64/fptrunc.ll
index 2187717c4148ae..b4c38e9f2df3b2 100644
--- a/llvm/test/CodeGen/AArch64/fptrunc.ll
+++ b/llvm/test/CodeGen/AArch64/fptrunc.ll
@@ -1,6 +1,7 @@
 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 3
 ; RUN: llc -mtriple=aarch64 -global-isel=0 -verify-machineinstrs %s -o - | FileCheck %s --check-prefixes=CHECK,CHECK-SD
 ; RUN: llc -mtriple=aarch64 -global-isel=1 -verify-machineinstrs %s -o - | FileCheck %s --check-prefixes=CHECK,CHECK-GI
+; RUN: llc -mtriple=aarch64 -global-isel=1 -mattr=+fullfp16,+bf16 -verify-machineinstrs %s -o - | FileCheck %s --check-prefixes=CHECK,CHECK-GI
 
 define float @fptrunc_f64_f32(double %a) {
 ; CHECK-LABEL: fptrunc_f64_f32:
diff --git a/llvm/utils/TableGen/GlobalISelEmitter.cpp b/llvm/utils/TableGen/GlobalISelEmitter.cpp
index 5466d315c05a49..2969dd9156ccbf 100644
--- a/llvm/utils/TableGen/GlobalISelEmitter.cpp
+++ b/llvm/utils/TableGen/GlobalISelEmitter.cpp
@@ -2346,6 +2346,20 @@ void GlobalISelEmitter::emitRunCustomAction(raw_ostream &OS) {
      << "}\n";
 }
 
+bool hasBFloatType(const TreePatternNode &Node) {
+  for (unsigned I = 0, E = Node.getNumTypes(); I < E; I++) {
+    auto Ty = Node.getType(I);
+    for (auto T : Ty)
+      if (T.second == MVT::bf16 ||
+          (T.second.isVector() && T.second.getScalarType() == MVT::bf16))
+        return true;
+  }
+  for (const TreePatternNode &C : Node.children())
+    if (hasBFloatType(C))
+      return true;
+  return false;
+}
+
 void GlobalISelEmitter::run(raw_ostream &OS) {
   if (!UseCoverageFile.empty()) {
     RuleCoverage = CodeGenCoverage();
@@ -2382,6 +2396,13 @@ void GlobalISelEmitter::run(raw_ostream &OS) {
 
     if (Pat.getGISelShouldIgnore())
       continue; // skip without warning
+
+    // Skip any patterns containing BF16 types, as GISel cannot currently tell
+    // the difference between fp16 and bf16. FIXME: This can be removed once
+    // BF16 is supported properly.
+    if (hasBFloatType(Pat.getSrcPattern()))
+      continue;
+
     auto MatcherOrErr = runOnPattern(Pat);
 
     // The pattern analysis can fail, indicating an unsupported pattern.