Re: [PATCH] preprocessor: c++: Support `#pragma GCC target' macros [PR87299]

[Jason Merrill via Gcc-patches]

On 7/31/23 22:22, Lewis Hyatt via Gcc-patches wrote:
`#pragma GCC target' is not currently handled in preprocess-only mode (e.g.,
when running gcc -E or gcc -save-temps). As noted in the PR, this means that
if the target pragma defines any macros, those macros are not effective in
preprocess-only mode. Similarly, such macros are not effective when
compiling with C++ (even when compiling without -save-temps), because C++
does not process the pragma until after all tokens have been obtained from
libcpp, at which point it is too late for macro expansion to take place.
Since r13-1544 and r14-2893, there is a general mechanism to handle pragmas
under these conditions as well, so resolve the PR by using the new "early
pragma" support.

toplev.cc required some changes because the target-specific handlers for
`#pragma GCC target' may call target_reinit(), and toplev.cc was not expecting
that function to be called in preprocess-only mode.

I added some additional testcases from the PR for x86. The other targets
that support `#pragma GCC target' (aarch64, arm, nios2, powerpc, s390)
already had tests verifying that the pragma sets macros as expected; here I
have added -save-temps to some of them, to test that it now works in
preprocess-only mode as well.

gcc/c-family/ChangeLog:

        PR preprocessor/87299
        * c-pragma.cc (init_pragma): Register `#pragma GCC target' and
        related pragmas in preprocess-only mode, and enable early handling.
        (c_reset_target_pragmas): New function refactoring code from...
        (handle_pragma_reset_options): ...here.
        * c-pragma.h (c_reset_target_pragmas): Declare.

gcc/cp/ChangeLog:

        PR preprocessor/87299
        * parser.cc (cp_lexer_new_main): Call c_reset_target_pragmas ()
        after preprocessing is complete, before starting compilation.

gcc/ChangeLog:

        PR preprocessor/87299
        * toplev.cc (no_backend): New static global.
        (finalize): Remove argument no_backend, which is now a
        static global.
        (process_options): Likewise.
        (do_compile): Likewise.
        (target_reinit): Don't do anything in preprocess-only mode.
        (toplev::main): Adapt to no_backend change.
        (toplev::finalize): Likewise.

gcc/testsuite/ChangeLog:

        PR preprocessor/87299
        * c-c++-common/pragma-target-1.c: New test.
        * c-c++-common/pragma-target-2.c: New test.
        * g++.target/i386/pr87299-1.C: New test.
        * g++.target/i386/pr87299-2.C: New test.
        * gcc.target/i386/pr87299-1.c: New test.
        * gcc.target/i386/pr87299-2.c: New test.
        * gcc.target/s390/target-attribute/tattr-2.c: Add -save-temps to the
        options, to test preprocess-only mode as well.
        * gcc.target/aarch64/pragma_cpp_predefs_1.c: Likewise.
        * gcc.target/arm/pragma_arch_attribute.c: Likewise.
        * gcc.target/nios2/custom-fp-2.c: Likewise.
        * gcc.target/powerpc/float128-3.c: Likewise.
---

Notes:
     Hello-
     
     This patch fixes the PR by enabling early pragma handling for `#pragma GCC
     target' and related pragmas such as `#pragma GCC push_options'. I did not
     need to touch any target-specific code, however I did need to make a change
     to toplev.cc, affecting all targets, to make it safe to call 
target_reinit()
     in preprocess-only mode. (Otherwise, it would be necessary to modify the
     implementation of target pragmas in every target, to avoid this code path.)
     That was the only complication I ran into.
     
     Regarding testing, I did: (thanks to GCC compile farm for the non-x86
     targets)
     
     bootstrap + regtest all languages - x86_64-pc-linux-gnu
     bootstrap + regtest c/c++ - powerpc64le-unknown-linux-gnu,
                                 aarch64-unknown-linux-gnu
     
     The following backends also implement this pragma so ought to be tested:
         arm
         nios2
         s390
     
     I am not able to test those directly. I did add coverage to their testsuites
     (basically, adding -save-temps to any existing test, causes it to test the
     pragma in preprocess-only mode.) Then, I verified on x86_64 with a cross
     compiler, that the modified testcases fail before the patch and pass
     afterwards. nios2 is an exception, it does not set any libcpp macros when
     handling the pragma, so there is nothing to test, but I did verify that
     processing the pragma in preprocess-only mode does not cause any problems.
     The cross compilers tested were targets arm-unknown-linux-gnueabi,
     nios2-unknown-linux, and s390-ibm-linux.
     
     Please let me know if it looks OK? Thanks!

The c-family and C++ changes look good.  David, any comments on the 
toplev changes?

Jason