https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92811
Bug ID: 92811
Summary: Odd optimisation differences with lambdas
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: david at westcontrol dot com
Target Milestone: ---
While playing with lambdas for higher order functions (in particular, functions
that manipulate functions and return new ones), I saw some differences in the
optimisations depending on seemingly minor changes to how functions are
defined. This is my sample code:
template <typename F>
auto twice(F f)
{
return [f](int v) { return f(f(v)); };
};
int g1(int i)
{
return i + 3;
};
auto g2 = [](int i) { return i + 3; };
int test(int i) {
auto tw = twice(g1);
return tw(i);
}
int test2(int i) {
auto tw = twice(g1);
auto tw2 = twice(tw);
return tw2(i);
}
int test3(int i) {
auto tw = twice(g2);
return tw(i);
}
int test4(int i) {
auto tw = twice(g2);
auto tw2 = twice(tw);
return tw2(i);
}
The generated assembly (x86-64) with -O2 or -O3 is:
g1(int):
leal 3(%rdi), %eax
ret
test(int):
leal 6(%rdi), %eax
ret
test2(int):
addl $6, %edi
call g1(int)
movl %eax, %edi
jmp g1(int)
test3(int):
leal 6(%rdi), %eax
ret
test4(int):
leal 12(%rdi), %eax
ret
With -O1, test3 and test4 (where the original function is a lambda) were still
fully optimised, while test1 and test2 had 2 and 4 calls to g1.
(For comparison, clang requires -O2 to get good code, but fully optimises all 4
test functions.)
