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| author | James E Wilson <wilson@specifixinc.com> | 2004-11-05 21:38:33 +0000 |
|---|---|---|
| committer | Jim Wilson <wilson@gcc.gnu.org> | 2004-11-05 13:38:33 -0800 |
| commit | ddd8e3ca2625ef3a1d15c363b79d02e9395b109a (patch) | |
| tree | a996ed86e9009ed6efba820c24b382189842e22d /gcc/doc | |
| parent | ea595d4ff3ce2b7ac10ceb665abd22bead716217 (diff) | |
| download | gcc-ddd8e3ca2625ef3a1d15c363b79d02e9395b109a.zip gcc-ddd8e3ca2625ef3a1d15c363b79d02e9395b109a.tar.gz gcc-ddd8e3ca2625ef3a1d15c363b79d02e9395b109a.tar.bz2 | |
Stop staying that -fno-guess-branch-probability produces random code changes.
* invoke.texi (-fno-guess-branch-probability): Rewrite.
From-SVN: r90144
Diffstat (limited to 'gcc/doc')
| -rw-r--r-- | gcc/doc/invoke.texi | 24 |
1 files changed, 11 insertions, 13 deletions
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi index eac209e..d6391dd 100644 --- a/gcc/doc/invoke.texi +++ b/gcc/doc/invoke.texi @@ -4758,19 +4758,17 @@ other, a few use both. @item -fno-guess-branch-probability @opindex fno-guess-branch-probability -Do not guess branch probabilities using a randomized model. - -Sometimes GCC will opt to use a randomized model to guess branch -probabilities, when none are available from either profiling feedback -(@option{-fprofile-arcs}) or @samp{__builtin_expect}. This means that -different runs of the compiler on the same program may produce different -object code. - -In a hard real-time system, people don't want different runs of the -compiler to produce code that has different behavior; minimizing -non-determinism is of paramount import. This switch allows users to -reduce non-determinism, possibly at the expense of inferior -optimization. +Do not guess branch probabilities using heuristics. + +GCC will use heuristics to guess branch probabilities if they are +not provided by profiling feedback (@option{-fprofile-arcs}). These +heuristics are based on the control flow graph. If some branch probabilities +are specified by @samp{__builtin_expect}, then the heuristics will be +used to guess branch probabilities for the rest of the control flow graph, +taking the @samp{__builtin_expect} info into account. The interactions +between the heuristics and @samp{__builtin_expect} can be complex, and in +some cases, it may be useful to disable the heuristics so that the effects +of @samp{__builtin_expect} are easier to understand. The default is @option{-fguess-branch-probability} at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}. |