On Mon, Jul 24, 2017 at 2:56 PM, Jan Hubicka <hubi...@ucw.cz> wrote:
> Hi,
> this patch adds -Wsuggest-attribute=cold because we can now statically detect
> cold functions atuomatically.
>
> Bootstrapped/regtested x86_64-linux. Plan to commit it tomorrow if there are
> no
> complains.
>
> Honza
>
> * invoke.texi (Wsuggest-attribute=cold): Document.
> * common.opt (Wsuggest-attribute=cold): New
> * ipa-pure-const.c (warn_function_cold): New function.
> * predict.c (compute_function_frequency): Use it.
> * predict.h (warn_function_cold): Declare.
>
> * gcc.dg/cold-1.c: New testcase.
Would it be possible to also do -Wsuggest-attribute=hot for symmetry's
sake? Just wondering.
> Index: common.opt
> ===================================================================
> --- common.opt (revision 250470)
> +++ common.opt (working copy)
> @@ -708,6 +708,10 @@ Wstrict-overflow=
> Common Joined RejectNegative UInteger Var(warn_strict_overflow) Warning
> Warn about optimizations that assume that signed overflow is undefined.
>
> +Wsuggest-attribute=cold
> +Common Var(warn_suggest_attribute_cold) Warning
> +Warn about functions which might be candidates for __attribute__((cold)).
> +
> Wsuggest-attribute=const
> Common Var(warn_suggest_attribute_const) Warning
> Warn about functions which might be candidates for __attribute__((const)).
> Index: doc/invoke.texi
> ===================================================================
> --- doc/invoke.texi (revision 250470)
> +++ doc/invoke.texi (working copy)
> @@ -5172,7 +5172,7 @@ whether to issue a warning. Similarly t
> setting of the option may result in warnings for benign code.
> @end table
>
> -@item -Wsuggest-attribute=@r{[}pure@r{|}const@r{|}noreturn@r{|}format@r{]}
> +@item
> -Wsuggest-attribute=@r{[}pure@r{|}const@r{|}noreturn@r{|}format@r{|}cold@r{]}
> @opindex Wsuggest-attribute=
> @opindex Wno-suggest-attribute=
> Warn for cases where adding an attribute may be beneficial. The
> @@ -5224,6 +5224,15 @@ might be appropriate for any function th
> @code{vprintf} or @code{vscanf}, but this might not always be the
> case, and some functions for which @code{format} attributes are
> appropriate may not be detected.
> +
> +@item -Wsuggest-attribute=cold
> +@opindex Wsuggest-attribute=cold
> +@opindex Wno-suggest-attribute=cold
> +
> +Warn about functions that might be candidates for @code{cold} attribute.
> This
> +is based on static detection and generally will only warn about functions
> which
> +always leads to a call to another @code{cold} function such as wrappers of
> +C++ @code{throw} or fatal error reporting functions leading to @code{abort}.
> @end table
>
> @item -Wsuggest-final-types
> Index: ipa-pure-const.c
> ===================================================================
> --- ipa-pure-const.c (revision 250470)
> +++ ipa-pure-const.c (working copy)
> @@ -232,6 +233,21 @@ warn_function_noreturn (tree decl)
> true, warned_about, "noreturn");
> }
>
> +void
> +warn_function_cold (tree decl)
> +{
> + tree original_decl = decl;
> +
> + cgraph_node *node = cgraph_node::get (decl);
> + if (node->instrumentation_clone)
> + decl = node->instrumented_version->decl;
> +
> + static hash_set<tree> *warned_about;
> + warned_about
> + = suggest_attribute (OPT_Wsuggest_attribute_cold, original_decl,
> + true, warned_about, "cold");
> +}
> +
> /* Return true if we have a function state for NODE. */
>
> static inline bool
> Index: predict.c
> ===================================================================
> --- predict.c (revision 250470)
> +++ predict.c (working copy)
> @@ -3611,7 +3611,10 @@ compute_function_frequency (void)
> if (ENTRY_BLOCK_PTR_FOR_FN (cfun)->count == profile_count::zero ()
> || lookup_attribute ("cold", DECL_ATTRIBUTES
> (current_function_decl))
> != NULL)
> - node->frequency = NODE_FREQUENCY_UNLIKELY_EXECUTED;
> + {
> + node->frequency = NODE_FREQUENCY_UNLIKELY_EXECUTED;
> + warn_function_cold (current_function_decl);
> + }
> else if (lookup_attribute ("hot", DECL_ATTRIBUTES
> (current_function_decl))
> != NULL)
> node->frequency = NODE_FREQUENCY_HOT;
> @@ -3630,7 +3633,10 @@ compute_function_frequency (void)
> Ipa-profile pass will drop functions only called from unlikely
> functions to unlikely and that is most of what we care about. */
> if (!cfun->after_inlining)
> - node->frequency = NODE_FREQUENCY_UNLIKELY_EXECUTED;
> + {
> + node->frequency = NODE_FREQUENCY_UNLIKELY_EXECUTED;
> + warn_function_cold (current_function_decl);
> + }
> FOR_EACH_BB_FN (bb, cfun)
> {
> if (maybe_hot_bb_p (cfun, bb))
> Index: predict.h
> ===================================================================
> --- predict.h (revision 250470)
> +++ predict.h (working copy)
> @@ -102,4 +102,7 @@ extern void propagate_unlikely_bbs_forwa
>
> extern void add_reg_br_prob_note (rtx_insn *, profile_probability);
>
> +/* In ipa-pure-const.c */
> +extern void warn_function_cold (tree);
> +
> #endif /* GCC_PREDICT_H */
> Index: testsuite/gcc.dg/cold-1.c
> ===================================================================
> --- testsuite/gcc.dg/cold-1.c (revision 0)
> +++ testsuite/gcc.dg/cold-1.c (working copy)
> @@ -0,0 +1,21 @@
> +/* { dg-do compile { target nonpic } } */
> +/* { dg-options "-O2 -Wsuggest-attribute=cold" } */
> +
> +extern void do_something_interesting_and_never_return ();
> +
> +int
> +foo1(int a)
> +{ /* { dg-warning "cold" "detect cold candidate" { target *-*-* } "8" } */
> + if (a)
> + abort ();
> + else
> + abort ();
> +}
> +
> +int
> +foo2(int a)
> +{
> + if (a)
> + do_something_interesting_and_never_return ();
> + abort ();
> +}
