On Thu, Aug 13, 2026 at 1:39 PM Jeffrey Law <[email protected]> wrote: > > > > On 8/12/2026 10:27 PM, Sam Price wrote: > > insert_value_copy_on_edge declares 'unsignedp' uninitialized and sets it > > only as a side effect of an argument to gcc_assert. When src_mode and > > dest_mode differ it is then passed to convert_modes, which uses it to > > choose between zero and sign extension. > > > > gcc_assert does not always evaluate its argument. With GCC_VERSION > > < 4005 and without assert checking, gcc/system.h defines > > > > #define gcc_assert(EXPR) ((void)(0 && (EXPR))) > > > > so promote_ssa_mode is never called and unsignedp is passed to > > convert_modes indeterminate, selecting zero or sign extension at random. > > GCC_VERSION is __GNUC__ * 1000 + __GNUC_MINOR__, so any host compiler > > reporting less than 4.5 takes this branch. > > > > Relying on a side effect in gcc_assert is against the coding > > conventions regardless of which expansion is in use. Call > > promote_ssa_mode outside the assert, as the same file already does in > > get_temp_reg, and keep the assert for the invariant alone. unsignedp is > > initialized for the !REG_P path, where promote_ssa_mode is not called. > > > > Signed-off-by: Sam Price<[email protected]> > > Assisted-by: Claude (Anthropic) > > --- > > > > Found while getting gcc.c-torture running against a MicroBlaze port to > > LLVM, comparing the two compilers over the suite. > > So I don't think is necessarily as simple as it might appear. In > particular what is the right value for the !REG case. I would suggest > one of two paths to make sure we're safe WRT the unsignedp initialization. > > First, make the call to promote_ssa_mode unconditionally. The worry > here is that it may not be ready to handle non-REG cases. > > Second, in the src_mode != dest_mode code, assert REG_P (dest). > > Given the change affects generic code it would need to be bootstrapped > and regression tested on one of the primary targets (x86, aarch64 linux > being the most common).
So the problem will only show up with --disable-checking and a non GCC build. Since GCC before 5.4.0 can't build GCC at this point. So maybe the best way forward is either removing assert checking or just changing the non-GCC gcc_assert case into the assert checking case. I filed https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126838 and Richard B. sugguests we remove --disable-checking. I tend to agree with him. Thanks, Andrea > > > Jeff
