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fix -O0 documentation
- From: gkeating at apple dot com (Geoffrey Keating)
- To: gcc-patches at gcc dot gnu dot org
- Date: Mon, 8 Jan 2007 23:29:05 -0800 (PST)
- Subject: fix -O0 documentation
The documentation for -O0 says 'do not optimize'. Although this is
partly true from a compiler implementation perspective, it isn't
helpful to users; it would imply that there is such a thing as
'unoptimized code', which is not the case, or at least GCC does not
generate it; GCC could certainly generate code that is much worse than
it does now even at -O0. It is also untrue in some ways, for example
GCC will allocate variables in registers at -O0 which is an
optimisation, and GCC will perform constant folding at -O0.
So, this patch replaces it with a summary of the wording just above,
which reads
> Without any optimization option, the compiler's goal is to reduce the
> cost of compilation and to make debugging produce the expected
> results. Statements are independent: if you stop the program with a
> breakpoint between statements, you can then assign a new value to any
> variable or change the program counter to any other statement in the
> function and get exactly the results you would expect from the source
> code.
which is much more correct, and is what the compiler actually does.
Bootstrapped on powerpc-darwin8.
--
- Geoffrey Keating <geoffk@apple.com>
===File ~/patches/gcc-doc-O0.patch==========================
2007-01-08 Geoffrey Keating <geoffk@apple.com>
* doc/invoke.texi (Optimize Options): Correct description of -O0.
Index: doc/invoke.texi
===================================================================
--- doc/invoke.texi (revision 120594)
+++ doc/invoke.texi (working copy)
@@ -4623,7 +4623,8 @@
@item -O0
@opindex O0
-Do not optimize. This is the default.
+Reduce compilation time and make debugging produce the expected
+results. This is the default.
@item -Os
@opindex Os
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