https://gcc.gnu.org/bugzilla/show_bug.cgi?id=41045
--- Comment #15 from GCC Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Jakub Jelinek <ja...@gcc.gnu.org>: https://gcc.gnu.org/g:ca4d6285974817080d3488b293c4970a8231372b commit r15-5932-gca4d6285974817080d3488b293c4970a8231372b Author: Jakub Jelinek <ja...@redhat.com> Date: Thu Dec 5 09:25:06 2024 +0100 Allow limited extended asm at toplevel [PR41045] In the Cauldron IPA/LTO BoF we've discussed toplevel asms and it was discussed it would be nice to tell the compiler something about what the toplevel asm does. Sure, I'm aware the kernel people said they aren't willing to use something like that, but perhaps other projects do. And for kernel perhaps we should add some new option which allows some dumb parsing of the toplevel asms and gather something from that parsing. The following patch is just a small step towards that, namely, allow some subset of extended inline asm outside of functions. The patch is unfinished, LTO streaming (out/in) of the ASM_EXPRs isn't implemented (it emits a sorry diagnostics), nor any cgraph/varpool changes to find out references etc. The patch allows something like: int a[2], b; enum { E1, E2, E3, E4, E5 }; struct S { int a; char b; long long c; }; asm (".section blah; .quad %P0, %P1, %P2, %P3, %P4; .previous" : : "m" (a), "m" (b), "i" (42), "i" (E4), "i" (sizeof (struct S))); Even for non-LTO, that could be useful e.g. for getting enumerators from C/C++ as integers into the toplevel asm, or sizeof/offsetof etc. The restrictions I've implemented are: 1) asm qualifiers aren't still allowed, so asm goto or asm inline can't be specified at toplevel, asm volatile has the volatile ignored for C++ with a warning and is an error in C like before 2) I see good use for mainly input operands, output maybe to make it clear that the inline asm may write some memory, I don't see a good use for clobbers, so the patch doesn't allow those (and of course labels because asm goto can't be specified) 3) the patch allows only constraints which don't allow registers, so typically "m" or "i" or other memory or immediate constraints; for memory, it requires that the operand is addressable and its address could be used in static var initializer (so that no code actually needs to be emitted for it), for others that they are constants usable in the static var initializers 4) the patch disallows + (there is no reload of the operands, so I don't see benefits of tying some operands together), nor % (who cares if something is commutative in this case), or & (again, no code is emitted around the asm), nor the 0-9 constraints Right now there is no way to tell the compiler that the inline asm defines some symbol, that is implemented in a later patch, as : constraint. Similarly, the c modifier doesn't work in all cases and the cc modifier is implemented separately. 2024-12-05 Jakub Jelinek <ja...@redhat.com> PR c/41045 gcc/ * output.h (insn_noperands): Declare. * final.cc (insn_noperands): No longer static. * varasm.cc (assemble_asm): Handle ASM_EXPR. * lto-streamer-out.cc (lto_output_toplevel_asms): Add sorry_at for non-STRING_CST toplevel asm for now. * doc/extend.texi (Basic @code{asm}, Extended @code{asm}): Document that extended asm is now allowed outside of functions with certain restrictions. gcc/c/ * c-parser.cc (c_parser_asm_string_literal): Add forward declaration. (c_parser_asm_definition): Parse also extended asm without clobbers/labels. * c-typeck.cc (build_asm_expr): Allow extended asm outside of functions and check extra restrictions. gcc/cp/ * cp-tree.h (finish_asm_stmt): Add TOPLEV_P argument. * parser.cc (cp_parser_asm_definition): Parse also extended asm without clobbers/labels outside of functions. * semantics.cc (finish_asm_stmt): Add TOPLEV_P argument, if set, check extra restrictions for extended asm outside of functions. * pt.cc (tsubst_stmt): Adjust finish_asm_stmt caller. gcc/testsuite/ * c-c++-common/toplevel-asm-1.c: New test. * c-c++-common/toplevel-asm-2.c: New test. * c-c++-common/toplevel-asm-3.c: New test.
