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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/doc | |
| 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/doc')
| -rw-r--r-- | gcc/doc/invoke.texi | 9 |
1 files changed, 8 insertions, 1 deletions
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi index de552b5..ca62157 100644 --- a/gcc/doc/invoke.texi +++ b/gcc/doc/invoke.texi @@ -588,7 +588,7 @@ Objective-C and Objective-C++ Dialects}. -fipa-ra -fvariable-expansion-in-unroller -fvect-cost-model -fvpt @gol -fweb -fwhole-program -fwpa -fuse-linker-plugin -fzero-call-used-regs @gol --param @var{name}=@var{value} --O -O0 -O1 -O2 -O3 -Os -Ofast -Og} +-O -O0 -O1 -O2 -O3 -Os -Ofast -Og -Oz} @item Program Instrumentation Options @xref{Instrumentation Options,,Program Instrumentation Options}. @@ -10734,6 +10734,13 @@ optimization flags except for those that may interfere with debugging: -fmove-loop-invariants -fmove-loop-stores -fssa-phiopt @gol -ftree-bit-ccp -ftree-dse -ftree-pta -ftree-sra} +@item -Oz +@opindex Oz +Optimize aggressively for size rather than speed. This may increase +the number of instructions executed if those instructions require +fewer bytes to encode. @option{-Oz} behaves similarly to @option{-Os} +including enabling most @option{-O2} optimizations. + @end table If you use multiple @option{-O} options, with or without level numbers, |