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. gcc/tree-outof-ssa.cc | 9 ++++++--- 1 file changed, 6 insertions(+), 3 deletions(-) diff --git a/gcc/tree-outof-ssa.cc b/gcc/tree-outof-ssa.cc index f4bc4878bd6ca..65fb2e16ea4ed 100644 --- a/gcc/tree-outof-ssa.cc +++ b/gcc/tree-outof-ssa.cc @@ -308,7 +308,7 @@ insert_value_copy_on_edge (edge e, int dest, tree src, location_t locus) { rtx dest_rtx, seq, x; machine_mode dest_mode, src_mode; - int unsignedp; + int unsignedp = 0; if (dump_file && (dump_flags & TDF_DETAILS)) { @@ -334,8 +334,11 @@ insert_value_copy_on_edge (edge e, int dest, tree src, location_t locus) src_mode = TYPE_MODE (TREE_TYPE (src)); dest_mode = GET_MODE (dest_rtx); gcc_assert (src_mode == TYPE_MODE (TREE_TYPE (name))); - gcc_assert (!REG_P (dest_rtx) - || dest_mode == promote_ssa_mode (name, &unsignedp)); + if (REG_P (dest_rtx)) + { + machine_mode pmode = promote_ssa_mode (name, &unsignedp); + gcc_assert (dest_mode == pmode); + } if (src_mode != dest_mode) {
