On Sat, 5 Sep 2020, Gary Oblock via Gcc wrote:
First off one of the questions just me being curious but second is quite serious. Note, this is GIMPLE coming into my optimization and not something I've modified.Here's the C code: type_t * do_comp( type_t *data, size_t len) { type_t *res; type_t *x = min_of_x( data, len); type_t *y = max_of_y( data, len); res = y; if ( x < y ) res = 0; return res; } And here's the resulting GIMPLE: ;; Function do_comp.constprop (do_comp.constprop.0, funcdef_no=5, decl_uid=4392, cgraph_uid=3, symbol_order=68) (executed once) do_comp.constprop (struct type_t * data) { struct type_t * res; struct type_t * x; struct type_t * y; size_t len; <bb 5> [local count: 1073741824]: <bb 2> [local count: 1073741824]: x_2 = min_of_x (data_1(D), 10000); y_3 = max_of_y (data_1(D), 10000); if (x_2 < y_3) goto <bb 3>; [29.00%] else goto <bb 4>; [71.00%] <bb 3> [local count: 311385128]: <bb 4> [local count: 1073741824]: # res_4 = PHI <y_3(2), 0B(3)> return res_4; } The silly question first. In the "if" stmt how does GCC get those probabilities? Which it shows as 29.00% and 71.00%. I believe they should both be 50.00%.
See the profile_estimate pass dump. One branch makes the function return NULL, which makes gcc guess that it may be a bit less likely than the other. Those are heuristics, which are tuned to help on average, but of course they are sometimes wrong.
The serious question is what is going on with this phi? res_4 = PHI <y_3(2), 0B(3)> This makes zero sense practicality wise to me and how is it supposed to be recognized and used? Note, I really do need to transform the "0B" into something else for my structure reorganization optimization.
That's not a question? Are you asking why PHIs exist at all? They are the standard way to represent merging in SSA representations. You can iterate on the PHIs of a basic block, etc.
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