[llvm] [llvm-reduce] Use poison for x86_amx default constants (PR #218157)

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 created https://github.com/llvm/llvm-project/pull/218157

This would otherwise cause an assertion due to executing an unreachable in getNullValue. The verifier also enforces that x86_amx types never have constants.

At some point we should probably move these types to being TargetExtTypes where we can probably more naturally express that we cannot have constants of this type, but that's a much larger undertaking.

>From ef261ed15377ec42588aebf6255a9206a6097112 Mon Sep 17 00:00:00 2001
From: Aiden Grossman <aidengrossman at google.com>
Date: Sat, 22 Aug 2026 19:51:35 +0000
Subject: [PATCH] [llvm-reduce] Avoid trying to create default constants for
 x86_amx

This would otherwise cause an assertion due to executing an unreachable
in getNullValue. The verifier also enforces that x86_amx types never
have constants.

At some point we should probably move these types to being
TargetExtTypes where we can probably more naturally express that we
cannot have constants of this type, but that's a much larger
undertaking.
---
 .../llvm-reduce/x86-amx-default-value.ll      | 73 +++++++++++++++++++
 llvm/tools/llvm-reduce/deltas/Utils.cpp       |  3 +
 2 files changed, 76 insertions(+)
 create mode 100644 llvm/test/tools/llvm-reduce/x86-amx-default-value.ll

diff --git a/llvm/test/tools/llvm-reduce/x86-amx-default-value.ll b/llvm/test/tools/llvm-reduce/x86-amx-default-value.ll
new file mode 100644
index 0000000000000..53fca54a1af5c
--- /dev/null
+++ b/llvm/test/tools/llvm-reduce/x86-amx-default-value.ll
@@ -0,0 +1,73 @@
+; RUN: llvm-reduce --delta-passes=instructions-to-return --test FileCheck --test-arg %s --test-arg --input-file %s
+
+; Check that we do not end up executing an unreachable by trying to create
+; a default constant value for an x86_amx value.
+
+; CHECK: x86_amx
+
+target triple = "x86_64-unknown-linux-gnu"
+
+define void @wobble() #0 {
+bbl:
+  %call = call x86_amx @llvm.x86.tilezero.internal(i16 0, i16 undef)
+  ret void
+}
+
+define <256 x i32> @ham() #0 {
+bbl:
+  %call = call x86_amx @llvm.x86.tileloadd64.internal(i16 undef, i16 0, ptr null, i64 0)
+  ret <256 x i32> zeroinitializer
+}
+
+define <256 x i32> @spam(i64 %arg, i1 %arg1) #0 {
+bbl:
+  %alloca = alloca i32, i32 4
+  br i1 %arg1, label %bbl2, label %bbl3
+
+bbl2:                                             ; preds = %bbl
+  call void @baz()
+  ret <256 x i32> zeroinitializer
+
+bbl3:                                             ; preds = %bbl
+  switch i64 %arg, label %bbl11 [
+    i64 2, label %bbl9
+    i64 1, label %bbl4
+  ]
+
+bbl4:                                             ; preds = %bbl3
+  call void @llvm.lifetime.start.p0(ptr %alloca)
+  call x86_amx @llvm.x86.tilezero.internal(i16 0, i16 undef)
+  call x86_amx @llvm.x86.tilezero.internal(i16 0, i16 undef)
+  br label %bbl5
+
+bbl5:                                             ; preds = %bbl5, %bbl4
+  %phi = phi ptr [ %alloca, %bbl4 ], [ null, %bbl5 ]
+  %phi6 = phi i1 [ false, %bbl4 ], [ true, %bbl5 ]
+  store volatile i32 0, ptr %phi, align 4
+  br i1 %phi6, label %bbl7, label %bbl5
+
+bbl7:                                             ; preds = %bbl5
+  %call = call <256 x i32> @ham()
+  %call8 = call <256 x i32> @ham()
+  ret <256 x i32> zeroinitializer
+
+bbl9:                                             ; preds = %bbl3
+  call x86_amx @llvm.x86.tilezero.internal(i16 0, i16 undef)
+  call x86_amx @llvm.x86.tilezero.internal(i16 0, i16 undef)
+  %call10 = call x86_amx @llvm.x86.tileloadd64.internal(i16 0, i16 0, ptr null, i64 0)
+  unreachable
+
+bbl11:                                            ; preds = %bbl3
+  call void @wobble()
+  ret <256 x i32> zeroinitializer
+}
+
+; Function Attrs: noreturn
+define void @baz() #3 {
+bbl:
+  call void @wobble()
+  unreachable
+}
+
+attributes #0 = { "target-features"="+amx-tile" }
+attributes #3 = { noreturn }
diff --git a/llvm/tools/llvm-reduce/deltas/Utils.cpp b/llvm/tools/llvm-reduce/deltas/Utils.cpp
index bc9c4c4d41948..0064a01a36c59 100644
--- a/llvm/tools/llvm-reduce/deltas/Utils.cpp
+++ b/llvm/tools/llvm-reduce/deltas/Utils.cpp
@@ -35,6 +35,9 @@ Value *llvm::getDefaultValue(Type *T) {
     return PoisonValue::get(TET);
   }
 
+  if (T->isX86_AMXTy())
+    return PoisonValue::get(T);
+
   return Constant::getNullValue(T);
 }