[llvm] [SPIRV] Error in backend for vararg functions (PR #169111)

[Nick Sarnie via llvm-commits]

https://github.com/sarnex created https://github.com/llvm/llvm-project/pull/169111

SPIR-V doesn't support variadic functions besides `printf`, and `printf` is already handled at this point.

If we don't do this, we generate invalid SPIR-V because the backend has no idea how to codegen vararg functions. We get asm like this:

```
%27 = OpFunction %6 None %7
%28 = OpFunctionParameter %4
                                        ; -- End function
```

The above is totally invalid and causes crashes in downstream tools like `spirv-as` and `spirv-link`.

We already have many `printf` tests locking down that this doesn't break `printf`.

Note the SPIR-V Translator does the same thing, see [here](https://github.com/KhronosGroup/SPIRV-LLVM-Translator/pull/2703).

>From fdd15b241bde3f4d0b742160330172cd1eefeae8 Mon Sep 17 00:00:00 2001
From: Nick Sarnie <nick.sarnie at intel.com>
Date: Fri, 21 Nov 2025 14:19:38 -0800
Subject: [PATCH] [SPIRV] Error in backend for vararg functions

Signed-off-by: Nick Sarnie <nick.sarnie at intel.com>
---
 llvm/lib/Target/SPIRV/SPIRVGlobalRegistry.cpp | 10 +++++++---
 llvm/lib/Target/SPIRV/SPIRVGlobalRegistry.h   |  2 +-
 llvm/test/CodeGen/SPIRV/function/vararg.ll    | 17 +++++++++++++++++
 3 files changed, 25 insertions(+), 4 deletions(-)
 create mode 100644 llvm/test/CodeGen/SPIRV/function/vararg.ll

diff --git a/llvm/lib/Target/SPIRV/SPIRVGlobalRegistry.cpp b/llvm/lib/Target/SPIRV/SPIRVGlobalRegistry.cpp
index 76fd834fd7219..b3f0478ecb8bf 100644
--- a/llvm/lib/Target/SPIRV/SPIRVGlobalRegistry.cpp
+++ b/llvm/lib/Target/SPIRV/SPIRVGlobalRegistry.cpp
@@ -970,8 +970,11 @@ SPIRVType *SPIRVGlobalRegistry::getOpTypeForwardPointer(
 }
 
 SPIRVType *SPIRVGlobalRegistry::getOpTypeFunction(
-    SPIRVType *RetType, const SmallVectorImpl<SPIRVType *> &ArgTypes,
+    const FunctionType *Ty, SPIRVType *RetType,
+    const SmallVectorImpl<SPIRVType *> &ArgTypes,
     MachineIRBuilder &MIRBuilder) {
+  if (Ty->isVarArg())
+    reportFatalUsageError("SPIR-V does not support variadic functions");
   return createOpType(MIRBuilder, [&](MachineIRBuilder &MIRBuilder) {
     auto MIB = MIRBuilder.buildInstr(SPIRV::OpTypeFunction)
                    .addDef(createTypeVReg(MIRBuilder))
@@ -988,7 +991,8 @@ SPIRVType *SPIRVGlobalRegistry::getOrCreateOpTypeFunctionWithArgs(
     MachineIRBuilder &MIRBuilder) {
   if (const MachineInstr *MI = findMI(Ty, false, &MIRBuilder.getMF()))
     return MI;
-  const MachineInstr *NewMI = getOpTypeFunction(RetType, ArgTypes, MIRBuilder);
+  const MachineInstr *NewMI =
+      getOpTypeFunction(cast<FunctionType>(Ty), RetType, ArgTypes, MIRBuilder);
   add(Ty, false, NewMI);
   return finishCreatingSPIRVType(Ty, NewMI);
 }
@@ -1097,7 +1101,7 @@ SPIRVType *SPIRVGlobalRegistry::createSPIRVType(
     for (const auto &ParamTy : FType->params())
       ParamTypes.push_back(findSPIRVType(ParamTy, MIRBuilder, AccQual,
                                          ExplicitLayoutRequired, EmitIR));
-    return getOpTypeFunction(RetTy, ParamTypes, MIRBuilder);
+    return getOpTypeFunction(FType, RetTy, ParamTypes, MIRBuilder);
   }
 
   unsigned AddrSpace = typeToAddressSpace(Ty);
diff --git a/llvm/lib/Target/SPIRV/SPIRVGlobalRegistry.h b/llvm/lib/Target/SPIRV/SPIRVGlobalRegistry.h
index c230e62e795e8..09c77f0cfd4f5 100644
--- a/llvm/lib/Target/SPIRV/SPIRVGlobalRegistry.h
+++ b/llvm/lib/Target/SPIRV/SPIRVGlobalRegistry.h
@@ -464,7 +464,7 @@ class SPIRVGlobalRegistry : public SPIRVIRMapping {
   SPIRVType *getOpTypeForwardPointer(SPIRV::StorageClass::StorageClass SC,
                                      MachineIRBuilder &MIRBuilder);
 
-  SPIRVType *getOpTypeFunction(SPIRVType *RetType,
+  SPIRVType *getOpTypeFunction(const FunctionType *Ty, SPIRVType *RetType,
                                const SmallVectorImpl<SPIRVType *> &ArgTypes,
                                MachineIRBuilder &MIRBuilder);
 
diff --git a/llvm/test/CodeGen/SPIRV/function/vararg.ll b/llvm/test/CodeGen/SPIRV/function/vararg.ll
new file mode 100644
index 0000000000000..a32b6fc541ad3
--- /dev/null
+++ b/llvm/test/CodeGen/SPIRV/function/vararg.ll
@@ -0,0 +1,17 @@
+; RUN: not llc -verify-machineinstrs -O0 -mtriple=spirv64-unknown-unknown --spirv-ext=+SPV_INTEL_function_pointers %s -o - 2>&1 | FileCheck %s
+
+ at glob = addrspace(1) global ptr null, align 8
+
+; Function Attrs: mustprogress noinline norecurse optnone
+define noundef i32 @main() #0 {
+entry:
+  %retval = alloca i32, align 4
+  %retval.ascast = addrspacecast ptr %retval to ptr addrspace(4)
+  store i32 0, ptr addrspace(4) %retval.ascast, align 4
+  store ptr @_Z3fooiz, ptr addrspace(4) addrspacecast (ptr addrspace(1) @glob to ptr addrspace(4)), align 8
+  call spir_func void (i32, ...) @_Z3fooiz(i32 noundef 5, i32 noundef 3)
+  ret i32 0
+}
+
+; CHECK: SPIR-V does not support variadic functions
+declare spir_func void @_Z3fooiz(i32 noundef, ...) #1