https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91841
Bug ID: 91841
Summary: vector_size(8) passes MMX register without emms
cleanup
Product: gcc
Version: 9.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: kretz at kde dot org
Target Milestone: ---
Target: i?86-*-*
Test case `g++ -O2 -m32` (cf. https://godbolt.org/z/RDUZo9):
#include <iostream>
using T = unsigned short;
using V [[gnu::vector_size(8)]] = T;
[[gnu::noinline]] V f(V x) { return x; }
int main() {
volatile float a = 1.f;
auto x = f(~V{});
asm("" :: "X"(x));
std::cout << a + 2.f;
}
The V parameter is passed via mm0 which leads to 1.f+2.f becoming -nan.
If GCC cannot reliably insert emms, it should not (never!) transparently emit
MMX code. Nowhere in this code did I ask for MMX... I understand that this
would be an ABI break. But at least clang does not implement this ABI either.
Why is this relevant?
namespace std {
template <class T, int N>
class simd {
using V [[gnu::vector_size(sizeof(T) * N)]] = T;
V d;
public:
...
};
}
Subsequently interleaving `std::simd<char, 8>` and any FPU code (forced e.g.
with long double) would lead to disaster.
