https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78847
Bug ID: 78847
Summary: pointer arithmetic from c++ ranged-based for loop not
optimized
Product: gcc
Version: 7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: krister.walfridsson at gmail dot com
Target Milestone: ---
GCC has some problems eliminating overhead from C++ range-based for loops.
Consider the program
#include <stddef.h>
#include <cstring>
#include <experimental/string_view>
using string_view = std::experimental::string_view;
class Foo {
constexpr static size_t Length = 9;
char ascii_[Length];
public:
Foo();
string_view view() const {
return string_view(ascii_, Length);
}
};
void testWithLoopValue(const Foo foo, size_t ptr, char *buf_) {
for (auto c : foo.view())
buf_[ptr++] = c;
}
compiled as
g++ -O3 -S -std=c++1z k.cpp
ldist determines that this is a memcpy of length expressed as _14
_18 = (unsigned long) &MEM[(void *)&foo + 9B];
_4 = &foo.ascii_ + 1;
_3 = (unsigned long) _4;
_16 = _18 + 1;
_14 = _16 - _3;
and dom3 improves this to
_18 = (unsigned long) &MEM[(void *)&foo + 9B];
_3 = (unsigned long) &MEM[(void *)&foo + 1B];
_16 = _18 + 1;
_14 = _16 - _3;
But this is not further simplified to 9 until combine, where it is too late,
and a call to memcpy is generated instead of the expected inlined version.
