On Sunday, 19 November 2023 22:53:37 CET Jan Hubicka wrote:
> Sadly it is really hard to work out this
> from IPA passes, since we basically care whether the iterator points to
> the same place as the end pointer, which are both passed by reference.
> This is inter-procedural value numbering that is quite out of reach.
I've done a fair share of branching on __builtin_constant_p in
std::experimental::simd to improve code-gen. It's powerful! But maybe we
also need the other side of the story to tell the optimizer: "I know you
can't const-prop everything; but this variable / expression, even if you
need to put in a lot of effort, the performance difference will be worth
it."
For std::vector, the remaining capacity could be such a value. The
functions f() and g() are equivalent (their code-gen isn't https://
compiler-explorer.com/z/r44ejK1qz):
#include <vector>
auto
f()
{
std::vector<int> x;
x.reserve(10);
for (int i = 0; i < 10; ++i)
x.push_back(0);
return x;
}
auto
g()
{ return std::vector<int>(10, 0); }
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Dr. Matthias Kretz https://mattkretz.github.io
GSI Helmholtz Center for Heavy Ion Research https://gsi.de
std::simd
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