[clang] [llvm] [MIPS][ISel] Fix musttail (PR #161860)

[Djordje Todorovic via cfe-commits]

djtodoro wrote:

> This PR doesn't fix (if it was intended to) sibling call optimizations.
> 
> It _does_ change the code generation for them, however - but it maintains all of the stack adjustments as well as the `fp`/`sp` moves.
> 
> ```c++
> __attribute__((noinline))
> int foo0()
> {
> 	volatile int a;
> 	return a += 1;
> }
> 
> int bar0()
> {
> 	return foo0();
> }
> ```
> 
> with tail calls disabled (`-std=gnu++23 -O3 -march=mips32r6 -fno-PIC -mllvm -mips-tail-calls=0`):
> 
> ```assembly
> 	addiu	$sp, $sp, -24
> 	sw	$ra, 20($sp)                    # 4-byte Folded Spill
> 	sw	$fp, 16($sp)                    # 4-byte Folded Spill
> 	move	$fp, $sp
> 	jal	_Z4foo0v
> 	nop
> 	move	$sp, $fp
> 	lw	$fp, 16($sp)                    # 4-byte Folded Reload
> 	lw	$ra, 20($sp)                    # 4-byte Folded Reload
> 	addiu	$sp, $sp, 24
> 	jrc	$ra
> ```
> 
> with tail calls enabled (`-std=gnu++23 -O3 -march=mips32r6 -fno-PIC -mllvm -mips-tail-calls=1`):
> 
> ```assembly
> 	addiu	$sp, $sp, -8
> 	sw	$ra, 4($sp)                     # 4-byte Folded Spill
> 	sw	$fp, 0($sp)                     # 4-byte Folded Spill
> 	move	$fp, $sp
> 	move	$sp, $fp
> 	lw	$fp, 0($sp)                     # 4-byte Folded Reload
> 	lw	$ra, 4($sp)                     # 4-byte Folded Reload
> 	j	_Z4foo0v
> 	addiu	$sp, $sp, 8
> ```
> 
> What is expected:
> 
> ```assembly
> 	j	_Z4foo0v
> 	nop
> ```
> 
> or
> 
> ```assembly
> 	bc	_Z4foo0v
> ```
> 
> So, it's moving the `j` to the end, but it's not cleaning up the stack changes that were _around_ the jump. Everything in here but the `j` could be removed (with a `nop`, but the `j` should be replaced with a `bc` anyways), but this effectively is generating a rather messed up function.
> 
> A significant amount of logic for handling this issue is present in `LowerCall` for PPC and for AArch64 (and the rest), but is missing in the MIPS version.

Hey @ameisen, thanks a lot. Yes, I agree, it is missing for MIPS case in the ISel Lowering, but I think this should be a separate github issue, and separate PR should be created for this. Can you please create an issue with the content from this comment? Thanks in advance.

https://github.com/llvm/llvm-project/pull/161860