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 for the worst.
```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`):
```asm
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`):
```asm
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
```
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.
https://github.com/llvm/llvm-project/pull/161860
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