loongarch64: failed to perform tail call elimination on a call site marked musttail

[folkertdev]

This LLVM IR

https://godbolt.org/z/8TsGoPvnj

target triple = "loongarch64-unknown-linux-gnu"

define internal void @callee(ptr dead_on_unwind noalias noundef nonnull writable writeonly sret([24 x i8]) align 1 captures(none) dereferenceable(24) initializes((0, 24)) %_0) unnamed_addr {
start:
  tail call void @llvm.memset.p0.i64(ptr noundef nonnull align 1 dereferenceable(24) %_0, i8 0, i64 24, i1 false)
  ret void
}

define void @dispatch(ptr dead_on_unwind noalias noundef writable writeonly sret([24 x i8]) align 1 captures(none) dereferenceable(24) initializes((0, 24)) %_0) unnamed_addr {
start:
  musttail call void @callee(ptr noalias noundef sret([24 x i8]) align 1 captures(none) dereferenceable(24) %_0)
  ret void
}

declare void @llvm.memset.p0.i64(ptr writeonly captures(none), i8, i64, i1 immarg) #1

Errors with

LLVM ERROR: failed to perform tail call elimination on a call site marked musttail

I suspect riscv has essentially the same problem, see also

• [clang] RISC-V: failed to perform tail call elimination on a call site marked musttail #157814
• riscv64: rustc-LLVM ERROR: failed to perform tail call elimination on a call site marked musttail rust-lang/rust#148748

This is inconvenient for rust stabilization of guaranteed tail calls. For rust it would be sufficient to accept the case where the stack space used for the return value is identical for callee and dispatch, that is probably easier to support than the general case.

[llvmbot]

@llvm/issue-subscribers-backend-loongarch

Author: Folkert de Vries (folkertdev)

This LLVM IR

https://godbolt.org/z/8TsGoPvnj

target triple = "loongarch64-unknown-linux-gnu"

define internal void @<!-- -->callee(ptr dead_on_unwind noalias noundef nonnull writable writeonly sret([24 x i8]) align 1 captures(none) dereferenceable(24) initializes((0, 24)) %_0) unnamed_addr {
start:
  tail call void @<!-- -->llvm.memset.p0.i64(ptr noundef nonnull align 1 dereferenceable(24) %_0, i8 0, i64 24, i1 false)
  ret void
}

define void @<!-- -->dispatch(ptr dead_on_unwind noalias noundef writable writeonly sret([24 x i8]) align 1 captures(none) dereferenceable(24) initializes((0, 24)) %_0) unnamed_addr {
start:
  musttail call void @<!-- -->callee(ptr noalias noundef sret([24 x i8]) align 1 captures(none) dereferenceable(24) %_0)
  ret void
}

declare void @<!-- -->llvm.memset.p0.i64(ptr writeonly captures(none), i8, i64, i1 immarg) #<!-- -->1

Errors with

LLVM ERROR: failed to perform tail call elimination on a call site marked musttail

I suspect riscv has essentially the same problem, see also

• [clang] RISC-V: failed to perform tail call elimination on a call site marked musttail #157814
• riscv64: rustc-LLVM ERROR: failed to perform tail call elimination on a call site marked musttail rust-lang/rust#148748

This is inconvenient for rust stabilization of guaranteed tail calls. For rust it would be sufficient to accept the case where the stack space used for the return value is identical for callee and dispatch, that is probably easier to support than the general case.

[folkertdev]

The same issue occurs when arguments don't fit in the registers and are passed via the stack

https://godbolt.org/z/G47z6rchK

target triple = "loongarch64-unknown-linux-gnu"

declare void @foo(i32,i32,i32,i32, i32,i32,i32,i32 ,i32)

define void @bar2(i32 %a0,i32 %a1,i32 %a2,i32 %a3,i32 %a4,i32 %a5,i32 %a6,i32 %a7,i32 %a8) nounwind {
  musttail call void @foo(i32 %a0,i32 %a1,i32 %a2,i32 %a3,i32 %a4,i32 %a5,i32 %a6,i32 %a7,i32 %a8)
  ret void
}

And again rust would only need support for the case where caller and callee use an identical amount of stack space.