[LLVM][MIPS] Sibling calls push/pop stack #165906
Description
At present, tail call optimizations do not work at all with or without the mips-tail-calls flag being set. This is related to and dependent upon, but distinct from, #161193.
This issue is being reported with PR #161860 by @djtodoro, as it makes tail calls functional at all in MIPS.
With said PR, sibling calls are not being optimized - the jump-link instructions is being pushed to the end and replaced with a jump and the jump-return removed, but the stack is still being set-up and torn-down.
__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):
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 $rawith tail calls enabled (-std=gnu++23 -O3 -march=mips32r6 -fno-PIC -mllvm -mips-tail-calls=1):
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, 8What is expected:
j _Z4foo0v
nopor
bc _Z4foo0vSo, 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, see #165634), 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.