Incorporate GCC9 fixes from upstream #2380
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And use it to replace the same pattern at the end of nlrthumb.c:nlr_jump.
Recent versions of gcc perform optimisations which can lead to the
following code from the MP_NLR_JUMP_HEAD macro being omitted:
top->ret_val = val; \
MP_NLR_RESTORE_PYSTACK(top); \
*_top_ptr = top->prev; \
This is noticeable (at least) in the unix coverage on x86-64 built with gcc
9.1.0. This is because the nlr_jump function is marked as no-return, so
gcc deduces that the above code has no effect.
Adding MP_UNREACHABLE tells the compiler that the asm code may branch
elsewhere, and so it cannot optimise away the code.
These s16-s21 registers are used by gcc so need to be saved. Future versions of gcc (beyond v9.1.0), or other compilers, may eventually need additional registers saved/restored. See issue micropython#4844.
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Yeah, this was a tricky one to track down. |
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Thanks to @dpgeorge for the fixes upstream. Thanks to @jerryneedell for finding the issue and @dhalbert for finding the related topics upstream.
Fixes #2378