diff options
Diffstat (limited to 'gcc')
| -rw-r--r-- | gcc/ChangeLog | 9 | ||||
| -rw-r--r-- | gcc/doc/invoke.texi | 45 |
2 files changed, 26 insertions, 28 deletions
diff --git a/gcc/ChangeLog b/gcc/ChangeLog index 36c0907..66a45c9 100644 --- a/gcc/ChangeLog +++ b/gcc/ChangeLog @@ -1,5 +1,14 @@ 2018-11-14 Sandra Loosemore <sandra@codesourcery.com> + PR lto/55102 + PR lto/56700 + * doc/invoke.texi (Optimize Options): Remove bad example about + interaction between -flto and -O. Replace it with a note that + you need to compile with -O and not just link. Copy-edit -flto + discussion to reduce verbiage and improve flow. + +2018-11-14 Sandra Loosemore <sandra@codesourcery.com> + PR middle-end/59658 * doc/invoke.texi (Optimize Options): Clarify that -O0 and -Og also suppress many optimizations. Alphabetize option lists for diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi index ed7ef00..f6486de 100644 --- a/gcc/doc/invoke.texi +++ b/gcc/doc/invoke.texi @@ -9822,15 +9822,11 @@ The above generates bytecode for @file{foo.c} and @file{bar.c}, merges them together into a single GIMPLE representation and optimizes them as usual to produce @file{myprog}. -The only important thing to keep in mind is that to enable link-time +The important thing to keep in mind is that to enable link-time optimizations you need to use the GCC driver to perform the link step. -GCC then automatically performs link-time optimization if any of the +GCC automatically performs link-time optimization if any of the objects involved were compiled with the @option{-flto} command-line option. -You generally -should specify the optimization options to be used for link-time -optimization though GCC tries to be clever at guessing an -optimization level to use from the options used at compile time -if you fail to specify one at link time. You can always override +You can always override the automatic decision to do link-time optimization by passing @option{-fno-lto} to the link command. @@ -9844,8 +9840,8 @@ the linker plugin is not available, @option{-fwhole-program} should be used to allow the compiler to make these assumptions, which leads to more aggressive optimization decisions. -When @option{-fuse-linker-plugin} is not enabled, when a file is -compiled with @option{-flto}, the generated object file is larger than +When a file is compiled with @option{-flto} without +@option{-fuse-linker-plugin}, the generated object file is larger than a regular object file because it contains GIMPLE bytecodes and the usual final code (see @option{-ffat-lto-objects}. This means that object files with LTO information can be linked as normal object @@ -9854,20 +9850,6 @@ interprocedural optimizations are applied. Note that when @option{-fno-fat-lto-objects} is enabled the compile stage is faster but you cannot perform a regular, non-LTO link on them. -Additionally, the optimization flags used to compile individual files -are not necessarily related to those used at link time. For instance, - -@smallexample -gcc -c -O0 -ffat-lto-objects -flto foo.c -gcc -c -O0 -ffat-lto-objects -flto bar.c -gcc -o myprog -O3 foo.o bar.o -@end smallexample - -This produces individual object files with unoptimized assembler -code, but the resulting binary @file{myprog} is optimized at -@option{-O3}. If, instead, the final binary is generated with -@option{-fno-lto}, then @file{myprog} is not optimized. - When producing the final binary, GCC only applies link-time optimizations to those files that contain bytecode. Therefore, you can mix and match object files and libraries with @@ -9875,15 +9857,22 @@ GIMPLE bytecodes and final object code. GCC automatically selects which files to optimize in LTO mode and which files to link without further processing. -There are some code generation flags preserved by GCC when -generating bytecodes, as they need to be used during the final link -stage. Generally options specified at link time override those -specified at compile time. +Generally, options specified at link time override those +specified at compile time, although in some cases GCC attempts to infer +link-time options from the settings used to compile the input files. If you do not specify an optimization level option @option{-O} at link time, then GCC uses the highest optimization level -used when compiling the object files. +used when compiling the object files. Note that it is generally +ineffective to specify an optimization level option only at link time and +not at compile time, for two reasons. First, compiling without +optimization suppresses compiler passes that gather information +needed for effective optimization at link time. Second, some early +optimization passes can be performed only at compile time and +not at link time. +There are some code generation flags preserved by GCC when +generating bytecodes, as they need to be used during the final link. Currently, the following options and their settings are taken from the first object file that explicitly specifies them: @option{-fPIC}, @option{-fpic}, @option{-fpie}, @option{-fcommon}, |