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

[via cfe-commits]

ameisen 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.

I'm trying to figure out how to word the issue, as without your PR clang doesn't even turn it into a tail call at all. Since sibling optimizations are based upon tail call optimizations (they're a subset of them) it complicates an issue as I'm not sure how to write an issue dependent upon a PR.

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