On 6/16/21 7:13 PM, Martin Sebor wrote:
On 6/11/21 6:53 AM, Martin Liška wrote:
Hello.
First, note that -O is equal to -O1 :) I noticed we don't use it consistently
in documentation:
$ git grep 'at.*-O1}' | cat
gcc/ada/gnat_ugn.texi:pick it based on the optimization level: 1 for
@code{-O1}, @code{-O2} or
...
Is the later (and more common variant) preferred?
I think -O1 should be preferred wherever the manual means to refer
to -O1. It's clearer that way mainly because other compilers have
different meanings for -O than GCC (e.g., in IBM XLC it corresponds
to -O2 and in Oracle C it's equivalent to level 3).
Thanks for clarification! Makes sense.
This is in line with the POSIX description of the c99 command which
leaves the -O level unspecified if none is provided.
Martin
Good, I'm going to apply the following patch.
Martin
>From 8eac92a07e386301f7b09f7ef6146e6e3ac6b6cd Mon Sep 17 00:00:00 2001
From: Martin Liska <mli...@suse.cz>
Date: Thu, 17 Jun 2021 11:48:59 +0200
Subject: [PATCH] docs: Use -O1 as a canonical value for -O option
gcc/ChangeLog:
* doc/invoke.texi: Use consistently -O1 instead of -O.
---
gcc/doc/invoke.texi | 74 ++++++++++++++++++++++-----------------------
1 file changed, 37 insertions(+), 37 deletions(-)
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index fe812cbd512..af2ce189fae 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -9944,7 +9944,7 @@ This option causes GCC to create markers in the internal representation
at the beginning of statements, and to keep them roughly in place
throughout compilation, using them to guide the output of @code{is_stmt}
markers in the line number table. This is enabled by default when
-compiling with optimization (@option{-Os}, @option{-O}, @option{-O2},
+compiling with optimization (@option{-Os}, @option{-O1}, @option{-O2},
@dots{}), and outputting DWARF 2 debug information at the normal level.
@item -gvariable-location-views
@@ -10223,7 +10223,7 @@ that do not involve a space-speed tradeoff.
As compared to @option{-O}, this option increases both compilation time
and the performance of the generated code.
-@option{-O2} turns on all optimization flags specified by @option{-O}. It
+@option{-O2} turns on all optimization flags specified by @option{-O1}. It
also turns on the following optimization flags:
@c Please keep the following list alphabetized!
@@ -10370,7 +10370,7 @@ instructions and checks if the result can be simplified. If loop unrolling
is active, two passes are performed and the second is scheduled after
loop unrolling.
-This option is enabled by default at optimization levels @option{-O},
+This option is enabled by default at optimization levels @option{-O1},
@option{-O2}, @option{-O3}, @option{-Os}.
@item -ffp-contract=@var{style}
@@ -10398,7 +10398,7 @@ Note that @option{-fno-omit-frame-pointer} doesn't guarantee the frame pointer
is used in all functions. Several targets always omit the frame pointer in
leaf functions.
-Enabled by default at @option{-O} and higher.
+Enabled by default at @option{-O1} and higher.
@item -foptimize-sibling-calls
@opindex foptimize-sibling-calls
@@ -10552,7 +10552,7 @@ This option is the default for optimized compilation if the assembler and
linker support it. Use @option{-fno-merge-constants} to inhibit this
behavior.
-Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
+Enabled at levels @option{-O1}, @option{-O2}, @option{-O3}, @option{-Os}.
@item -fmerge-all-constants
@opindex fmerge-all-constants
@@ -10638,7 +10638,7 @@ long} on a 32-bit system, split the registers apart and allocate them
independently. This normally generates better code for those types,
but may make debugging more difficult.
-Enabled at levels @option{-O}, @option{-O2}, @option{-O3},
+Enabled at levels @option{-O1}, @option{-O2}, @option{-O3},
@option{-Os}.
@item -fsplit-wide-types-early
@@ -10750,18 +10750,18 @@ Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
@opindex fauto-inc-dec
Combine increments or decrements of addresses with memory accesses.
This pass is always skipped on architectures that do not have
-instructions to support this. Enabled by default at @option{-O} and
+instructions to support this. Enabled by default at @option{-O1} and
higher on architectures that support this.
@item -fdce
@opindex fdce
Perform dead code elimination (DCE) on RTL@.
-Enabled by default at @option{-O} and higher.
+Enabled by default at @option{-O1} and higher.
@item -fdse
@opindex fdse
Perform dead store elimination (DSE) on RTL@.
-Enabled by default at @option{-O} and higher.
+Enabled by default at @option{-O1} and higher.
@item -fif-conversion
@opindex fif-conversion
@@ -10770,7 +10770,7 @@ includes use of conditional moves, min, max, set flags and abs instructions, and
some tricks doable by standard arithmetics. The use of conditional execution
on chips where it is available is controlled by @option{-fif-conversion2}.
-Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}, but
+Enabled at levels @option{-O1}, @option{-O2}, @option{-O3}, @option{-Os}, but
not with @option{-Og}.
@item -fif-conversion2
@@ -10778,7 +10778,7 @@ not with @option{-Og}.
Use conditional execution (where available) to transform conditional jumps into
branch-less equivalents.
-Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}, but
+Enabled at levels @option{-O1}, @option{-O2}, @option{-O3}, @option{-Os}, but
not with @option{-Og}.
@item -fdeclone-ctor-dtor
@@ -10958,7 +10958,7 @@ If supported for the target machine, attempt to reorder instructions
to exploit instruction slots available after delayed branch
instructions.
-Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os},
+Enabled at levels @option{-O1}, @option{-O2}, @option{-O3}, @option{-Os},
but not at @option{-Og}.
@item -fschedule-insns
@@ -11196,7 +11196,7 @@ and the @option{large-stack-frame-growth} parameter to 400.
@item -ftree-reassoc
@opindex ftree-reassoc
Perform reassociation on trees. This flag is enabled by default
-at @option{-O} and higher.
+at @option{-O1} and higher.
@item -fcode-hoisting
@opindex fcode-hoisting
@@ -11219,7 +11219,7 @@ enabled by default at @option{-O3}.
@item -ftree-forwprop
@opindex ftree-forwprop
Perform forward propagation on trees. This flag is enabled by default
-at @option{-O} and higher.
+at @option{-O1} and higher.
@item -ftree-fre
@opindex ftree-fre
@@ -11227,12 +11227,12 @@ Perform full redundancy elimination (FRE) on trees. The difference
between FRE and PRE is that FRE only considers expressions
that are computed on all paths leading to the redundant computation.
This analysis is faster than PRE, though it exposes fewer redundancies.
-This flag is enabled by default at @option{-O} and higher.
+This flag is enabled by default at @option{-O1} and higher.
@item -ftree-phiprop
@opindex ftree-phiprop
Perform hoisting of loads from conditional pointers on trees. This
-pass is enabled by default at @option{-O} and higher.
+pass is enabled by default at @option{-O1} and higher.
@item -fhoist-adjacent-loads
@opindex fhoist-adjacent-loads
@@ -11244,24 +11244,24 @@ by default at @option{-O2} and higher.
@item -ftree-copy-prop
@opindex ftree-copy-prop
Perform copy propagation on trees. This pass eliminates unnecessary
-copy operations. This flag is enabled by default at @option{-O} and
+copy operations. This flag is enabled by default at @option{-O1} and
higher.
@item -fipa-pure-const
@opindex fipa-pure-const
Discover which functions are pure or constant.
-Enabled by default at @option{-O} and higher.
+Enabled by default at @option{-O1} and higher.
@item -fipa-reference
@opindex fipa-reference
Discover which static variables do not escape the
compilation unit.
-Enabled by default at @option{-O} and higher.
+Enabled by default at @option{-O1} and higher.
@item -fipa-reference-addressable
@opindex fipa-reference-addressable
Discover read-only, write-only and non-addressable static variables.
-Enabled by default at @option{-O} and higher.
+Enabled by default at @option{-O1} and higher.
@item -fipa-stack-alignment
@opindex fipa-stack-alignment
@@ -11282,14 +11282,14 @@ cold functions are marked as cold. Also functions executed once (such as
@code{cold}, @code{noreturn}, static constructors or destructors) are
identified. Cold functions and loop less parts of functions executed once are
then optimized for size.
-Enabled by default at @option{-O} and higher.
+Enabled by default at @option{-O1} and higher.
@item -fipa-modref
@opindex fipa-modref
Perform interprocedural mod/ref analysis. This optimization analyzes the side
effects of functions (memory locations that are modified or referenced) and
enables better optimization across the function call boundary. This flag is
-enabled by default at @option{-O} and higher.
+enabled by default at @option{-O1} and higher.
@item -fipa-cp
@opindex fipa-cp
@@ -11417,7 +11417,7 @@ currently enabled, but may be enabled by @option{-O2} in the future.
@item -ftree-sink
@opindex ftree-sink
Perform forward store motion on trees. This flag is
-enabled by default at @option{-O} and higher.
+enabled by default at @option{-O1} and higher.
@item -ftree-bit-ccp
@opindex ftree-bit-ccp
@@ -11431,14 +11431,14 @@ It requires that @option{-ftree-ccp} is enabled.
@opindex ftree-ccp
Perform sparse conditional constant propagation (CCP) on trees. This
pass only operates on local scalar variables and is enabled by default
-at @option{-O} and higher.
+at @option{-O1} and higher.
@item -fssa-backprop
@opindex fssa-backprop
Propagate information about uses of a value up the definition chain
in order to simplify the definitions. For example, this pass strips
sign operations if the sign of a value never matters. The flag is
-enabled by default at @option{-O} and higher.
+enabled by default at @option{-O1} and higher.
@item -fssa-phiopt
@opindex fssa-phiopt
@@ -11464,7 +11464,7 @@ be limited using @option{max-tail-merge-comparisons} parameter and
@item -ftree-dce
@opindex ftree-dce
Perform dead code elimination (DCE) on trees. This flag is enabled by
-default at @option{-O} and higher.
+default at @option{-O1} and higher.
@item -ftree-builtin-call-dce
@opindex ftree-builtin-call-dce
@@ -11489,26 +11489,26 @@ Perform a variety of simple scalar cleanups (constant/copy
propagation, redundancy elimination, range propagation and expression
simplification) based on a dominator tree traversal. This also
performs jump threading (to reduce jumps to jumps). This flag is
-enabled by default at @option{-O} and higher.
+enabled by default at @option{-O1} and higher.
@item -ftree-dse
@opindex ftree-dse
Perform dead store elimination (DSE) on trees. A dead store is a store into
a memory location that is later overwritten by another store without
any intervening loads. In this case the earlier store can be deleted. This
-flag is enabled by default at @option{-O} and higher.
+flag is enabled by default at @option{-O1} and higher.
@item -ftree-ch
@opindex ftree-ch
Perform loop header copying on trees. This is beneficial since it increases
effectiveness of code motion optimizations. It also saves one jump. This flag
-is enabled by default at @option{-O} and higher. It is not enabled
+is enabled by default at @option{-O1} and higher. It is not enabled
for @option{-Os}, since it usually increases code size.
@item -ftree-loop-optimize
@opindex ftree-loop-optimize
Perform loop optimizations on trees. This flag is enabled by default
-at @option{-O} and higher.
+at @option{-O1} and higher.
@item -ftree-loop-linear
@itemx -floop-strip-mine
@@ -11663,7 +11663,7 @@ in such a way that its value when exiting the loop can be determined using
only its initial value and the number of loop iterations, replace uses of
the final value by such a computation, provided it is sufficiently cheap.
This reduces data dependencies and may allow further simplifications.
-Enabled by default at @option{-O} and higher.
+Enabled by default at @option{-O1} and higher.
@item -fivopts
@opindex fivopts
@@ -11705,13 +11705,13 @@ Perform temporary expression replacement during the SSA->normal phase. Single
use/single def temporaries are replaced at their use location with their
defining expression. This results in non-GIMPLE code, but gives the expanders
much more complex trees to work on resulting in better RTL generation. This is
-enabled by default at @option{-O} and higher.
+enabled by default at @option{-O1} and higher.
@item -ftree-slsr
@opindex ftree-slsr
Perform straight-line strength reduction on trees. This recognizes related
expressions involving multiplications and replaces them by less expensive
-calculations when possible. This is enabled by default at @option{-O} and
+calculations when possible. This is enabled by default at @option{-O1} and
higher.
@item -ftree-vectorize
@@ -11892,7 +11892,7 @@ The default is @option{-fguess-branch-probability} at levels
Reorder basic blocks in the compiled function in order to reduce number of
taken branches and improve code locality.
-Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
+Enabled at levels @option{-O1}, @option{-O2}, @option{-O3}, @option{-Os}.
@item -freorder-blocks-algorithm=@var{algorithm}
@opindex freorder-blocks-algorithm
@@ -11903,7 +11903,7 @@ or @samp{stc}, the ``software trace cache'' algorithm, which tries to
put all often executed code together, minimizing the number of branches
executed by making extra copies of code.
-The default is @samp{simple} at levels @option{-O}, @option{-Os}, and
+The default is @samp{simple} at levels @option{-O1}, @option{-Os}, and
@samp{stc} at levels @option{-O2}, @option{-O3}.
@item -freorder-blocks-and-partition
@@ -12473,7 +12473,7 @@ explicit comparison operation.
This pass only applies to certain targets that cannot explicitly represent
the comparison operation before register allocation is complete.
-Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
+Enabled at levels @option{-O1}, @option{-O2}, @option{-O3}, @option{-Os}.
@item -fcprop-registers
@opindex fcprop-registers
@@ -12481,7 +12481,7 @@ After register allocation and post-register allocation instruction splitting,
perform a copy-propagation pass to try to reduce scheduling dependencies
and occasionally eliminate the copy.
-Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
+Enabled at levels @option{-O1}, @option{-O2}, @option{-O3}, @option{-Os}.
@item -fprofile-correction
@opindex fprofile-correction
--
2.32.0
