https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90271
Bug ID: 90271
Summary: [missed-optimization] failure to keep variables in
registers during "faux" memcpy
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: eyalroz at technion dot ac.il
Target Milestone: ---
Example on GodBolt: https://godbolt.org/z/Q17L1u
Consider the following functions:
template<typename T1, typename T2>
inline void replace_bytes (T1& v1 ,const T2& v2 ,std::size_t k) noexcept
{
if (k > sizeof(T1) - sizeof(T2)) { return; }
std::memcpy( (void*) (((char*)&v1)+k) , (const void*) &v2 , sizeof(T2) );
}
For plain-old-data types, this is nothing but the manipulation of v1's bytes
(and there are no pointer aliasing issues). So, at least when k is known at
compile-time, the compiler should IMHO keep the activity to within registers.
And yet - GCC doesn't: With the extra code
int foo1()
{
int x = 3;
char c = 1;
replace_bytes(x,c,1);
return x;
}
we get (at maximum optimization):
foo1():
mov DWORD PTR [rsp-4], 3
mov BYTE PTR [rsp-3], 1
mov eax, DWORD PTR [rsp-4]
ret
This, while clang _does_ optimize fully and has foo1() simply return 259 (=
256+3).
Even if we make k a template parameter - it doesn't help.
