Cranelift: Incorrect abi for i128, i128 return value on x86_64 sysv

[bjorn3]

Expected Results

All return values are returned using a single return pointer.

Actual Results

Return values are put into registers where possible and on the stack for the rest.

Versions and Environment

Cranelift version or commit: 0.111

Operating system: Linux

Architecture: x86_64

Extra Info

This is one of the causes for rust-lang/rustc_codegen_cranelift#1525. The same issue may exist on aarch64 and other archs too, but haven't checked yet.

[bjorn3]

PR's opened while working on this:

• Couple of cleanups to the ABI computation #9253
• Remove StructArgument support from the arm64, riscv64 and s390x backends #9258
• Couple of improvements to the abi handling code (part 3) #9267
• Couple of improvements to the abi handling code (part 4) #9284
• Make the Tail call conv follow the system call conv for the return area ptr #9287

[bjorn3]

Found another issue: The x86_64 and riscv64 backends add an implicit return pointer after all other arguments rather than as first argument.

Edit: Fixed in #9267

[bjorn3]

Expected Results

All return values are returned using a single return pointer.

Actually, LLVM passes the return value on the stack if it doesn't fit in registers. It doesn't use a return area pointer.

Edit: Never mind. I was wrong about this.

[bjorn3]

As of rust-lang/rust#131211 rustc should no longer emit this return type. If this is indeed the case, I propose to deny any return type that doesn't fit in the abi designated return value registers for at least the system_v and windows_fastcall call convs. And ideally also for every other call conv to be able to get rid of the implicit sret argument introduction in the abi calculation code which complicates this code by a fair bit.