If-conversion misses many cases

["Jan Hoogerbrugge" <hoogerbrugge at hotmail dot com>]

Hi,

My target architecture supports predication so I expressed that in
my machine description. This is what I added:

       (define_attr "predicable" "no,yes" (const_string "yes"))

       (define_cond_exec
               [(match_operator 0 "predicate_operator"
                       [(match_operand:BI 1 "register_operand" "r") 
(const_int 0)])]
               ""
               "if %J0"
       )

       int predicate_operator(rtx op, enum machine_mode mode)
       {
               enum rtx_code code = GET_CODE(op);

               return (GET_MODE(op) == mode || mode == VOIDmode) &&
                      (code == EQ || code == NE);
       }

       #define MAX_CONDITIONAL_EXECUTE                 12

I see some if=conversion happening but I don't see it happening in
many simple cases where I would expect it. In the following set of
test cases it only happens in foo4.

       int s;
       int t;

       void foo(int a)          { if(a > 0) s = 0; }
       void foo2(int a)         { if(a == 0) s = 0; }
       void foo3(int a)         { if(a == 3) s = 0; else t = 0; }
       void foo4(int a, int *p) { if(a >= 4) *p = 1;  }
       void foo5(int a, int *p) { if(a == 3) *p = 0; if(a == 0) s = 0; }
       void foo6(int a, int *p) { if(a == 3) s = 0; else *p = 2; }
       void foo7(int a, int *p) { if(a & 1) s = 0; else *p = 2; }
       void foo8(int a, int *p) { if((a & 1) == 0) s = 0; else *p = 2; }

The IA64 port applies if-conversion in all these cases. So I am
wondering why I am missing so many opportunities for if-conversion.

When I compare execution traces for my architecture and IA64 for
this case

       int s;

       void foo(int a) { int *p = &s; if(a > 0) *p = 0; }

I see differences in the modified_in_p (test, insn) test in
cond_exec_process_insns. For IA64, this tests returns false, for my target
(TriMedia) it returns true, which causes that must_be_last is set and
that causes that cond_exec_process_insns returns false. I don't understand
what is going on here so I would appreciate some help to find out why
if-conversion is so infrequently happening. Anybody who can help me?

Thanks,
Jan

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