[Bug target/32803] -Os: shorter load immediates for x86_64

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=32803

--- Comment #9 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Roger Sayle <sayle@gcc.gnu.org>:

https://gcc.gnu.org/g:e742722f76c70be303248da7ca4842198d4fd1cc

commit r12-6057-ge742722f76c70be303248da7ca4842198d4fd1cc
Author: Roger Sayle <roger@nextmovesoftware.com>
Date:   Sat Dec 18 13:47:52 2021 +0000

    PR target/32803: Add -Oz option for improved clang compatibility.

    This patch adds support for an -Oz command line option, aggressively
    optimizing for size at the expense of performance.  GCC's current -Os
    provides a reasonable balance of size and performance, whereas -Oz is
    probably only useful for code size benchmarks such as CSiBE.  Or so I
    thought until I read in https://news.ycombinator.com/item?id=25408853
    that clang's -Oz sometimes outperforms -O[23s]; I suspect modern
instruction
    decode stages can treat "pushq $1; popq %rax" as a short uop encoding.

    Instead of introducing a new global variable, this patch simply abuses
    the existing optimize_size by setting its value to 2.  The only change
    in behaviour is the tweak to the i386 backend implementing the suggestion
    in PR target/32803 to use a short push/pop sequence for loading small
    immediate values (-128..127) on x86, matching the behaviour of LLVM.

    On x86_64, the simple function:
    int foo() { return 25; }

    currently generates with -Os:
    foo:    movl    $25, %eax       // 5 bytes
            ret

    With the proposed -Oz, it generates:
    foo:    pushq   $25             // 2 bytes
            popq    %rax            // 1 byte
            ret

    On CSiBE, this results in a 0.94% improvement (3703513 bytes total
    down to 3668516 bytes).

    2021-12-18  Roger Sayle  <roger@nextmovesoftware.com>

    gcc/ChangeLog
            PR target/32803
            * common.opt (Oz): New command line option.
            * doc/invoke.texi: Document the new -Oz option.
            * lto-wrapper.c (merge_and_complain, append_compiler_options):
            Treat OPT_Oz as synonymous with OPT_Os.
            * optc-save-gen.awk: Increase maximum value of optimize_size to 2.
            * opts.c (default_options_optimization) [OPT_Oz]: Handle OPT_Oz
            just like OPT_Os, except set opt->x_optimize_size to 2.
            (common_handle_option): Skip OPT_Oz just like OPT_Os.

            * config/i386/i386.md (*movdi_internal): Use a push/pop sequence
            for suitable SImode TYPE_IMOV moves when optimize_size > 1.
            (*movsi_internal): Likewise.

    gcc/testsuite/ChangeLog
            PR target/32803
            * gcc.target/i386/pr32803.c: New test case.