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| author | Roger Sayle <roger@nextmovesoftware.com> | 2021-12-18 13:47:52 +0000 |
|---|---|---|
| committer | Roger Sayle <roger@nextmovesoftware.com> | 2021-12-18 13:47:52 +0000 |
| commit | e742722f76c70be303248da7ca4842198d4fd1cc (patch) | |
| tree | caf44d6eb01f547b4cb58c4d158ddc475bfe4971 /gcc/common.opt | |
| parent | cc032ec1ecb34b006f42e170ccb9d76aa42fd8eb (diff) | |
| download | gcc-e742722f76c70be303248da7ca4842198d4fd1cc.zip gcc-e742722f76c70be303248da7ca4842198d4fd1cc.tar.gz gcc-e742722f76c70be303248da7ca4842198d4fd1cc.tar.bz2 | |
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.
Diffstat (limited to 'gcc/common.opt')
| -rw-r--r-- | gcc/common.opt | 4 |
1 files changed, 4 insertions, 0 deletions
diff --git a/gcc/common.opt b/gcc/common.opt index 4af89ce..2ed818d 100644 --- a/gcc/common.opt +++ b/gcc/common.opt @@ -483,6 +483,10 @@ Og Common Optimization Optimize for debugging experience rather than speed or size. +Oz +Common Optimization +Optimize for space aggressively rather than speed. + Q Driver |