* builtins.c (expand_builtin_expect): New.

        (expand_builtin): Call it.
        * builtins.def (BUILT_IN_EXPECT): New.
        * c-common.c (c_common_nodes_and_builtins): Declare __builtin_expect.
        * extend.texi: Document it.

        * predict.c (expected_value_to_br_prob): New.
        (find_expected_value): New.
        * basic-block.h (expected_value_to_br_prob): Declare.
        * toplev.c (rest_of_compilation): Invoke it.

        * rtl.h (NOTE_EXPECTED_VALUE): New.
        (NOTE_INSN_EXPECTED_VALUE): New.
        * rtl.c (note_insn_name): Update.
        * print-rtl.c (print_rtx): Reorg NOTE_LINE_NUMBER special
        cases; handle NOTE_INSN_EXPECTED_VALUE.


git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@33211 138bc75d-0d04-0410-961f-82ee72b054a4

1 files changed, 35 insertions, 0 deletions

diff --git a/gcc/extend.texi b/gcc/extend.texi
index 22fe7414d2e..36c451b8485 100644
--- a/gcc/extend.texi
+++ b/gcc/extend.texi
@@ -3199,7 +3199,9 @@ correspond to the C library functions @code{abort}, @code{abs},
 @code{sinf}, @code{sinl}, @code{sqrt}, @code{sqrtf}, @code{sqrtl},
 @code{strcmp}, @code{strcpy}, and @code{strlen}.
 
+@table @code
 @findex __builtin_constant_p
+@item __builtin_constant_p (@var{exp})
 You can use the builtin function @code{__builtin_constant_p} to
 determine if a value is known to be constant at compile-time and hence
 that GNU CC can perform constant-folding on expressions involving that
@@ -3228,6 +3230,39 @@ or constructor expression (@pxref{Constructors}) and will not return 1
 when you pass a constant numeric value to the inline function unless you
 specify the @samp{-O} option.
 
+@findex __builtin_expect
+@item __builtin_expect(@var{exp}, @var{c})
+You may use @code{__builtin_expect} to provide the compiler with 
+branch prediction information.  In general, you should prefer to
+use actual profile feedback for this (@samp{-fprofile-arcs}), as
+programmers are notoriously bad at predicting how their programs
+actually preform.  However, there are applications in which this
+data is hard to collect.
+
+The return value is the value of @var{exp}, which should be an
+integral expression.  The value of @var{c} must be a compile-time
+constant.  The semantics of the builtin are that it is expected
+that @var{exp} == @var{c}.  For example:
+
+@smallexample
+if (__builtin_expect (x, 0))
+  foo ();
+@end smallexample
+
+@noindent
+would indicate that we do not expect to call @code{foo}, since
+we expect @code{x} to be zero.  Since you are limited to integral
+expressions for @var{exp}, you should use constructions such as
+
+@smallexample
+if (__builtin_expect (ptr != NULL, 1))
+  error ();
+@end smallexample
+
+@noindent
+when testing pointer or floating-point values.
+@end table
+
 @node Deprecated Features
 @section Deprecated Features