https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109205
Bug ID: 109205
Summary: vector.resize( v.size() + 100 ) does unnecessary
comparison
Product: gcc
Version: 12.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: joerg.rich...@pdv-fs.de
Target Milestone: ---
This function:
#include <vector>
void testResize( std::vector<char> & v )
{
v.resize( v.size() + 100 );
}
Will compile (-O2) to something like this:
mov rcx, QWORD PTR [rdi+8]
mov rax, QWORD PTR [rdi]
mov rdx, rcx
sub rdx, rax
lea rsi, [rdx+100]
cmp rdx, rsi
jb .L39
add rax, rsi
cmp rcx, rax
je .L36
mov QWORD PTR [rdi+8], rax
.L36:
ret
.L39:
mov esi, 100
jmp std::vector<char, std::allocator<char>
>::_M_default_append(unsigned long)
The call to _M_default_append is guarded by a cmp. This seems unnecessary, as
the argument passed to resize is always bigger than size.
Doing the addition with a signed type does not change the result.
See: https://godbolt.org/z/xY77fsz4z
