On Tue, Nov 13, 2018 at 9:20 AM Martin Liška <[email protected]> wrote:
>
> On 11/13/18 5:43 AM, Jason Merrill wrote:
> > [[likely]] and [[unlikely]] are equivalent to the GNU hot/cold attributes,
> > except that they can be applied to arbitrary statements as well as labels;
> > this is most likely to be useful for marking if/else branches as likely or
> > unlikely. Conveniently, PREDICT_EXPR fits the bill nicely as a
> > representation.
> >
> > I also had to fix marking case labels as hot/cold, which didn't work before.
> > Which then required me to force __attribute ((fallthrough)) to apply to the
> > statement rather than the label.
> >
> > Tested x86_64-pc-linux-gnu. Does this seem like a sane implementation
> > approach to people with more experience with PREDICT_EXPR?
>
> Hi.
>
> In general it makes sense to implement it the same way. Question is how much
> should the hold/cold attribute should be close to __builtin_expect.
>
> Let me present few examples and differences that I see:
>
> 1) ./xgcc -B. -O2 -fdump-tree-profile_estimate=/dev/stdout /tmp/test1.C
>
> ;; Function foo (_Z3foov, funcdef_no=0, decl_uid=2301, cgraph_uid=1,
> symbol_order=3)
>
> Predictions for bb 2
> first match heuristics: 90.00%
> combined heuristics: 90.00%
> __builtin_expect heuristics of edge 2->3: 90.00%
>
> As seen here __builtin_expect is stronger as it's first match heuristics and
> has probability == 90%.
>
> ;; Function bar (_Z3barv, funcdef_no=1, decl_uid=2303, cgraph_uid=2,
> symbol_order=4)
>
> Predictions for bb 2
> DS theory heuristics: 74.48%
> combined heuristics: 74.48%
> opcode values nonequal (on trees) heuristics of edge 2->3: 34.00%
> hot label heuristics of edge 2->3: 85.00%
>
> Here we combine hot label prediction with the opcode one, resulting in quite
> poor result 75%.
> So maybe cold/hot prediction cal also happen first match.
Makes sense.
> 2) ./xgcc -B. -O2 -fdump-tree-profile_estimate=/dev/stdout /tmp/test2.C
> ...
> foo ()
> {
> ...
> switch (_3) <default: <L3> [3.33%], case 3: <L0> [3.33%], case 42: <L1>
> [3.33%], case 333: <L2> [90.00%]>
>
> while:
>
> bar ()
> {
> switch (a.1_1) <default: <L3> [25.00%], case 3: <L0> [25.00%], case 42:
> <L1> [25.00%], case 333: <L2> [25.00%]>
> ...
>
> Note that support for __builtin_expect was enhanced in this stage1. I can
> definitely cover also situations when one uses
> hot/cold for labels. So definitely place for improvement.
Hmm, the gimplifier should be adding a PREDICT_EXPR for the case label
now, is it just ignored currently?
> 3) last example where one can use the attribute for function decl, resulting
> in:
> __attribute__((hot, noinline))
> foo ()
> {
> ..
>
> Hope it's desired? If so I would cover that with a test-case in test-suite.
[[likely]] and [[unlikely]] don't apply to functions; I suppose I
should diagnose that.
> Jason can you please point to C++ specification of the attributes?
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0479r5.html
> Would you please consider an error diagnostics for situations written in
> test4.C?
A warning seems appropriate. You think the front end is the right
place for that?
Jason
