yronglin wrote: > There's one more questionable effect of this commit (together with the fix in > #89567). Some of our code started triggering "loop not unrolled" warnings. A > reduced example is at https://godbolt.org/z/jG4xsGY7s > > ``` > #include <cstddef> > #include <bit> > > template <bool Flag> > int FailToBuild(int n) { > constexpr int N = 100; > auto init = [=]() { return Flag ? n : 0UL; }; > auto cond = [=](size_t ix) { return Flag ? ix != 0 : ix < 10; }; > auto iter = [=](size_t ix) { > return Flag ? ix & ~(1ULL << std::countr_zero(ix)) : ix + 1; > }; > #pragma unroll Flag ? 1 : N > for (size_t ix = init(); cond(ix); ix = iter(ix)) { > n *= n; > } > return n; > } > > int foo(int n) { > return FailToBuild<true>(n); > } > > int bar(int n) { > return FailToBuild<false>(n); > } > ``` > > `clang -std=c++20 -O3` started generating the following warning on this code > after the commit: > > ``` > <source>:13:3: warning: loop not unrolled: the optimizer was unable to > perform the requested transformation; the transformation might be disabled or > specified as part of an unsupported transformation ordering > [-Wpass-failed=transform-warning] > 13 | for (size_t ix = init(); cond(ix); ix = iter(ix)) { > ``` > > It doesn't seem the generated code has actually changed, thus, the question > is whether these are legit warnings that used to be missed, or vice versa?
> There's one more questionable effect of this commit (together with the fix in > #89567). Some of our code started triggering "loop not unrolled" warnings. A > reduced example is at https://godbolt.org/z/jG4xsGY7s > > ``` > #include <cstddef> > #include <bit> > > template <bool Flag> > int FailToBuild(int n) { > constexpr int N = 100; > auto init = [=]() { return Flag ? n : 0UL; }; > auto cond = [=](size_t ix) { return Flag ? ix != 0 : ix < 10; }; > auto iter = [=](size_t ix) { > return Flag ? ix & ~(1ULL << std::countr_zero(ix)) : ix + 1; > }; > #pragma unroll Flag ? 1 : N > for (size_t ix = init(); cond(ix); ix = iter(ix)) { > n *= n; > } > return n; > } > > int foo(int n) { > return FailToBuild<true>(n); > } > > int bar(int n) { > return FailToBuild<false>(n); > } > ``` > > `clang -std=c++20 -O3` started generating the following warning on this code > after the commit: > > ``` > <source>:13:3: warning: loop not unrolled: the optimizer was unable to > perform the requested transformation; the transformation might be disabled or > specified as part of an unsupported transformation ordering > [-Wpass-failed=transform-warning] > 13 | for (size_t ix = init(); cond(ix); ix = iter(ix)) { > ``` > > It doesn't seem the generated code has actually changed, thus, the question > is whether these are legit warnings that used to be missed, or vice versa? Thank you for your report, I will find out the cause as soon as possible. https://github.com/llvm/llvm-project/pull/88666 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
