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 <sa...@gcc.gnu.org>: https://gcc.gnu.org/g:e742722f76c70be303248da7ca4842198d4fd1cc commit r12-6057-ge742722f76c70be303248da7ca4842198d4fd1cc Author: Roger Sayle <ro...@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 <ro...@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.
