[llvm-bugs] [Bug 40232] New: [MergeFuncs] Thunk calls may be invalid under calling convention

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=40232

            Bug ID: 40232
           Summary: [MergeFuncs] Thunk calls may be invalid under calling
                    convention
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Interprocedural Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: nikita.ppv at gmail.com
                CC: llvm-bugs at lists.llvm.org

Originally reported at https://github.com/rust-lang/rust/pull/57268.

Running

define ptx_kernel i32 @func1(i32 %a, i32 %b) {
  %x = add i32 %a, %b
  %y = add i32 %x, %b
  ret i32 %y
}
define ptx_kernel i32 @func2(i32 %a, i32 %b) {
  %x = add i32 %a, %b
  %y = add i32 %x, %b
  ret i32 %y
}

through opt -mergefunc results in

define ptx_kernel i32 @func1(i32 %a, i32 %b) {
  %x = add i32 %a, %b
  %y = add i32 %x, %b
  ret i32 %y
}

define ptx_kernel i32 @func2(i32, i32) {
  %3 = tail call ptx_kernel i32 @func1(i32 %0, i32 %1)
  ret i32 %3
}

However, while LLVM does not validate this an error, calling a ptx_kernel
function from another ptx_kernel function is not legal and will error while
assembling.

Leaving aside the question of whether merging functions on the NVPTX target
makes sense, the correct way to merge these functions is probably the same as
for interposable functions:

define i32 @0(i32 %a, i32 %b) {
  %x = add i32 %a, %b
  %y = add i32 %x, %b
  ret i32 %y
}

define ptx_kernel i32 @func1(i32, i32) {
  %3 = tail call i32 @0(i32 %0, i32 %1)
  ret i32 %y
}

define ptx_kernel i32 @func2(i32, i32) {
  %3 = tail call i32 @0(i32 %0, i32 %1)
  ret i32 %3
}

Apart from ptx_kernel, there are probably other cases that are affected by this
as well (I'd imagine amdgpu_kernel and spir_kernel may have similar
restrictions.)

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