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).
This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
and make -k check with no new failures. Ok for mainline?
2021-12-10 Roger Sayle
gcc/ChangeLog
PR target/32803
* common.opt (Oz): New command line 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.
Thanks in advance,
Roger
--
diff --git a/gcc/common.opt b/gcc/common.opt
index fa0a44f..bf6aed2 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
diff --git a/gcc/lto-wrapper.c b/gcc/lto-wrapper.c
index 54f642d..7d2b7e5 100644
--- a/gcc/lto-wrapper.c
+++ b/gcc/lto-wrapper.c
@@ -370,12 +370,14 @@ merge_and_complain (vec
_options,
case OPT_Ofast:
case OPT_Og:
case OPT_Os:
+ case OPT_Oz:
existing_opt = -1;
for (j = 0; j < decoded_options.length (); ++j)
if (decoded_options[j].opt_index == OPT_O
|| decoded_options[j].opt_index == OPT_Ofast
|| decoded_options[j].opt_index == OPT_Og
- || decoded_options[j].opt_index == OPT_Os)
+ || decoded_options[j].opt_index == OPT_Os
+ || decoded_options[j].opt_index == OPT_Oz)
{
existing_opt = j;
break;
@@ -407,6 +409,7 @@ merge_and_complain (vec _options,
level = MAX (level, 1);
break;
case OPT_Os:
+ case OPT_Oz:
level = MAX (level, 2);
break;
default:
@@ -428,6 +431,7 @@ merge_and_complain (vec _options,
level = MAX (level, 1);
break;
case OPT_Os:
+ case OPT_Oz:
level = MAX (level, 2);
break;
default:
@@ -725,6 +729,7 @@ append_compiler_options (obstack *argv_obstack,
vec opts)
case OPT_Ofast:
case OPT_Og:
case OPT_Os:
+ case OPT_Oz:
break;
case OPT_Xassembler:
diff --git a/gcc/optc-save-gen.awk b/gcc/optc-save-gen.awk
index e363ac7..81c2db0 100644
--- a/gcc/optc-save-gen.awk
+++ b/gcc/optc-save-gen.awk
@@ -93,7 +93,7 @@ var_opt_char[1] = "optimize_size";
var_opt_char[2] = "optimize_debug";
var_opt_char[3] = "optimize_fast";
var_opt_range["optimize"] = "0, 255";
-var_opt_range["optimize_size"] = "0, 1";
+var_opt_range["optimize_size"] = "0, 2";
var_opt_range["optimize_debug"] = "0, 1";
var_opt_range["optimize_fast"] = "0, 1";
diff --git a/gcc/opts.c b/gcc/opts.c
index b16497e..7fba3d7a 100644
--- a/gcc/opts.c
+++ b/gcc/opts.c
@@ -745,6 +745,15 @@ default_options_optimization (struct gcc_options *opts,
opts->x_optimize_debug = 0;
break;
+ case OPT_Oz:
+ opts->x_optimize_size = 2;
+
+ /* Optimizing for size forces optimize to be 2. */
+ opts->x_optimize = 2;
+ opts->x_optimize_fast = 0;
+ opts->x_optimize_debug = 0;
+ break;
+
