Repository navigation
musttail doesn't optimize sibling calls #147813
Description
For example, this code:
int bar(int);
int
foo_tail(int x)
{
// Correct, compiles to a single jmp:
return bar(x);
}
int
foo_musttail(int x)
{
// Buggy, loads and re-spills all parameters:
__attribute__((musttail)) return bar(x);
}Compiles to:
_foo_tail:
jmp _bar
_foo_musttail:
movl 4(%esp), %eax
movl %eax, 4(%esp)
jmp _barAdding more parameters to the function (and tail call) increases the number of redundant loads/spills, too.
Activity
llvmbot commentedon Jul 10, 2025
@llvm/issue-subscribers-backend-x86
Author: Brandt Bucher (brandtbucher)
Details
For example, this code:Compiles to:
Compiler Explorer
Adding more parameters to the function (and tail call) increases the number of redundant loads/spills, too.
[-]`musttail` generates redundant moves on 32-bit platforms[/-][+]`musttail` doesn't optimize sibling calls[/+]brandtbucher commentedon Jul 22, 2025
Actually, this isn't a 32-bit issue, it's just easier to trigger with fewer parameters there. Here's the same thing on 64-bit: https://godbolt.org/z/1cao7Gxhv
Turns out
musttailjust doesn't optimize sibling calls. Patch incoming.Hendiadyoin1 commentedon Jul 31, 2025
I am getting this, or a closely related issue in c++ land and a member-pointer sibling call which does not consistently
result in a tail-call, without a diagnostic at that
(LadybirdBrowser/ladybird#5641 or the thread here on the ladybird discord as a reference)
I was not yet able to make a minimal reproducer,
but I can confirm that the linked patch does not suffice
brandtbucher commentedon Jul 31, 2025
Is your issue that tail-calls aren't being emitted at all? That seems like something separate.
This is just a code quality issue, not a correctness one.
Hendiadyoin1 commentedon Jul 31, 2025
ah, yeah it fully fails to tail-call in some cases, seems like I misread the issue (multiple times)
x86: fix musttail sibcall miscompilation (llvm#168956)
Automerge: x86: fix musttail sibcall miscompilation (#168956)
x86: fix musttail sibcall miscompilation (llvm#168956)
Automerge: x86: fix musttail sibcall miscompilation (#168956)
x86: fix musttail sibcall miscompilation (#168956)
x86: fix musttail sibcall miscompilation (#168956)
x86: fix musttail sibcall miscompilation (#168956)