I've checked in this patch to address the complaints about a bad example
in the discussion of -flto reported in PR55102 and its duplicate
PR56700. The bad example implied that you can compile files at -O0 and
then link them with -O3 to get full optimization, which is not correct.
I replaced that with some language explaining why you need to compile
with optimization to get the full effect of LTO.
I also did a bit of copy-editing and rearrangement of material in the
-flto discussion so that it flows a little better and isn't quite as wordy.
-Sandra
2018-11-14 Sandra Loosemore <san...@codesourcery.com>
PR lto/55102
PR lto/56700
gcc/
* 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.
Index: gcc/doc/invoke.texi
===================================================================
--- gcc/doc/invoke.texi (revision 266162)
+++ gcc/doc/invoke.texi (working copy)
@@ -9822,15 +9822,11 @@ The above generates bytecode for @file{f
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, @opt
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 applie
@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.
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},
