On Sun, Feb 16, 2025 at 4:38 PM Jeff Law <jeffreya...@gmail.com> wrote: > > This is Jakub's patch and Ian's testcase for the slightly vexing fault > building the D runtime with an s390x-x-riscv cross compiler. > > The core issue is we're allocating a vector to hold temporary registers > unconditionally, including cases where the vector isn't needed because > the loop isn't going to iterate. > > In the cases where the vector isn't needed the length is computed with > an expression (x / y) - 1 where x / y will be zero. The alloca(-1) on > the s390 platform triggers a fault. We haven't seen the fault with an > x86 cross, but we can certainly see the bogus value being passed to > alloca with a debugger.
I would expect alloca(-1) to trigger a fault with -fstack-clash-protection even on x86, so we should indeed avoid doing this. > > Jakub patch just conditionalizes the whole block in a sensible way. So > it looks larger than it really is. I thought it might be better to do a > bit of manual CSE on this code to make it even more obvious, but I think > we're ultimately OK here. > > Ian provided the testcase, collapsed down into equivalent C code. > Again, it doesn't fault on an x86-x-riscv, but I can see the incorrect > behavior with a debugger. > > And a shout-out to Stefan for providing a docker based reproducer, it > really helped track this down. > > Waiting for the pre-commit tester to do its thing before committing. > > Jeff >
