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Re: Add -Wsuggest-attribute=cold


On Mon, Jul 24, 2017 at 2:56 PM, Jan Hubicka <hubicka@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 ();
> +}


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