GCC Middle and Back End API Reference
tree-ssa-loop-ch.c File Reference


static bool should_duplicate_loop_header_p (basic_block header, struct loop *loop, int *limit)
static bool do_while_loop_p ()
static unsigned int copy_loop_headers ()
static bool gate_ch ()
gimple_opt_passmake_pass_ch ()

Function Documentation

static unsigned int copy_loop_headers ( )
   For all loops, copy the condition at the end of the loop body in front
   of the loop.  This is beneficial since it increases efficiency of
   code motion optimizations.  It also saves one jump on entry to the loop.  
         Copy at most 20 insns.  
         If the loop is already a do-while style one (either because it was
         written as such, or because jump threading transformed it into one),
         we might be in fact peeling the first iteration of the loop.  This
         in general is not a good idea.  
         Iterate the header copying up to limit; this takes care of the cases
         like while (a && b) {...}, where we want to have both of the conditions
         copied.  TODO -- handle while (a || b) - like cases, by not requiring
         the header to have just a single successor and copying up to
             Find a successor of header that is inside a loop; i.e. the new
             header after the condition is copied.  
         Ensure that the header will have just the latch as a predecessor
         inside the loop.  
         If the loop has the form "for (i = j; i < j + 10; i++)" then
         this copying can introduce a case where we rely on undefined
         signed overflow to eliminate the preheader condition, because
         we assume that "j < j + 10" is true.  We don't want to warn
         about that case for -Wstrict-overflow, because in general we
         don't warn about overflow involving loops.  Prevent the
         warning by setting the no_warning flag in the condition.  
         Ensure that the latch and the preheader is simple (we know that they
         are not now, since there was the loop exit condition.  
static bool do_while_loop_p ( )
   Checks whether LOOP is a do-while style loop.  
     If the latch of the loop is not empty, it is not a do-while loop.  
     If the header contains just a condition, it is not a do-while loop.  
static bool gate_ch ( )
gimple_opt_pass* make_pass_ch ( )
static bool should_duplicate_loop_header_p ( basic_block  header,
struct loop loop,
int *  limit 

Loop header copying on trees. Copyright (C) 2004-2013 Free Software Foundation, Inc.

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   Duplicates headers of loops if they are small enough, so that the statements
   in the loop body are always executed when the loop is entered.  This
   increases effectiveness of code motion optimizations, and reduces the need
   for loop preconditioning.  
   Check whether we should duplicate HEADER of LOOP.  At most *LIMIT
   instructions should be duplicated, limit is decreased by the actual
     Do not copy one block more than once (we do not really want to do
     loop peeling here).  
     Loop header copying usually increases size of the code.  This used not to
     be true, since quite often it is possible to verify that the condition is
     satisfied in the first iteration and therefore to eliminate it.  Jump
     threading handles these cases now.  
     If this is not the original loop header, we want it to have just
     one predecessor in order to match the && pattern.  
     Approximately copy the conditions that used to be used in jump.c --
     at most 20 insns and no calls.