https://gcc.gnu.org/bugzilla/show_bug.cgi?id=42612
Dmitry Baksheev <bd at mail dot ru> changed:
What |Removed |Added
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CC| |bd at mail dot ru
--- Comment #6 from Dmitry Baksheev <bd at mail dot ru> ---
Please consider fixing this issue. Here is another example where not using
post-increment for loops produces suboptimal code on AArch64. The code is 4x
slower than LLVM-generated code for dot-product function:
double dotprod(std::size_t n,
const double* __restrict__ a,
const double* __restrict__ b)
{
double ans = 0;
#if __clang__
#pragma clang loop vectorize(assume_safety)
#else
#pragma GCC ivdep
#endif
for (std::size_t i = 0; i < n; ++i) {
ans += a[i] * b[i];
}
return ans;
}
Compile with: $(CXX) -march=armv8.2-a -O3 dp.cpp
GCC-generated loop does not have post-increment loads:
.L3:
ldr d2, [x1, x3, lsl 3]
ldr d1, [x2, x3, lsl 3]
add x3, x3, 1
fmadd d0, d2, d1, d0
cmp x0, x3
bne .L3
Clang emits this:
.LBB0_4:
ldr d1, [x10], #8
ldr d2, [x8], #8
subs x9, x9, #1
fmadd d0, d1, d2, d0
b.ne .LBB0_4
