[llvm-branch-commits] [llvm] release/23.x: [ExpandVariadics] Don't lower unmangled C printf on SPIR-V (#206998) (PR #213893)

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llvmorg-github-actions[bot] wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: llvmbot

<details>
<summary>Changes</summary>

Backport b1d21c6d3121898dc13c0df8cf9e4410bee75cfb

Requested by: @<!-- -->pvelesko

---
Full diff: https://github.com/llvm/llvm-project/pull/213893.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/IPO/ExpandVariadics.cpp (+6-1) 
- (added) llvm/test/Transforms/ExpandVariadics/spirv-printf.ll (+35) 


``````````diff
diff --git a/llvm/lib/Transforms/IPO/ExpandVariadics.cpp b/llvm/lib/Transforms/IPO/ExpandVariadics.cpp
index 4a45e8d5137a4..948752718fde6 100644
--- a/llvm/lib/Transforms/IPO/ExpandVariadics.cpp
+++ b/llvm/lib/Transforms/IPO/ExpandVariadics.cpp
@@ -1067,8 +1067,13 @@ struct SPIRV final : public VariadicABIInfo {
     StringRef DemangledName(Demangled);
 
     // Skip any SPIR-V builtins.
+    // Note: an unmangled C `printf` declaration demangles to "printf" with no
+    // argument list, so the "printf(" prefix check below misses it. Match the
+    // bare name as well so OpenCL/HIP printf (emitted unmangled) is left as a
+    // variadic call for the backend's OpenCL.std printf lowering to expand
+    // inline, rather than being packed into a vararg buffer here.
     if (DemangledName.starts_with("__spirv_") ||
-        DemangledName.starts_with("printf("))
+        DemangledName.starts_with("printf(") || F->getName() == "printf")
       return true;
 
     return false;
diff --git a/llvm/test/Transforms/ExpandVariadics/spirv-printf.ll b/llvm/test/Transforms/ExpandVariadics/spirv-printf.ll
new file mode 100644
index 0000000000000..c9be77a405243
--- /dev/null
+++ b/llvm/test/Transforms/ExpandVariadics/spirv-printf.ll
@@ -0,0 +1,35 @@
+; RUN: opt -mtriple=spirv64-unknown-unknown -S --passes=expand-variadics --expand-variadics-override=lowering %s | FileCheck %s
+;
+; An unmangled C `printf` declaration demangles to bare "printf" (no argument
+; list). On SPIR-V it must be left as a variadic call so the backend can lower
+; it to the OpenCL.std printf ExtInst with inline operands, instead of having
+; its arguments packed into a vararg buffer here. A user-defined variadic
+; function with a body is still expanded as usual.
+
+ at .str = private unnamed_addr addrspace(2) constant [4 x i8] c"%d\0A\00"
+
+declare spir_func i32 @printf(ptr addrspace(2), ...)
+
+; printf is left untouched: the call stays variadic and nothing is packed
+; (CHECK-NEXT pins that ret immediately follows the call).
+; CHECK-LABEL: define spir_kernel void @uses_printf(
+; CHECK-NEXT:    {{%.*}} = call spir_func i32 (ptr addrspace(2), ...) @printf(ptr addrspace(2) @.str, i32 42)
+; CHECK-NEXT:    ret void
+define spir_kernel void @uses_printf() {
+  %r = call spir_func i32 (ptr addrspace(2), ...) @printf(ptr addrspace(2) @.str, i32 42)
+  ret void
+}
+
+; Control: an ordinary variadic function with a body is still expanded, i.e.
+; its caller packs the arguments into a vararg buffer.
+define spir_func i32 @my_variadic(...) {
+  ret i32 0
+}
+
+; CHECK-LABEL: define spir_kernel void @uses_my_variadic(
+; CHECK:    %vararg_buffer = alloca %uses_my_variadic.vararg
+; CHECK:    call spir_func i32 @my_variadic(ptr %vararg_buffer)
+define spir_kernel void @uses_my_variadic() {
+  %r = call spir_func i32 (...) @my_variadic(i32 7)
+  ret void
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/213893