Help with clz(0) and optimization

[Jeff Law]

On 10/31/23 08:26, Enrico via Gcc wrote:
> I am working on GCC for a target architecture where clz(0) is defined and
> is 32 (TriCore).
> The code under test is the following:
> 
> #include <stdio.h>
> int f(unsigned int a) {
>      unsigned int res = 32 - __builtin_clz(a);
>      if(res > 0) printf("test");
>      return res;
> }
> 
>  From GCC version greater and equal to 11, the Tree SSA optimization
> discards the if condition completely, and always executes the printf. This
> is the optimized gimplified code at the end of the optimization for GCC12:
> 
> int f (unsigned int a)
> {
>    int _1;
>    int _2;
> 
>    <bb 2> [local count: 1073741824]:
>    _1 = __builtin_clz (a_4(D));
>    _2 = 32 - _1;
>    __printf_chk (1, "test");
>    return _2;
> }
> 
> where GCC10 produces:
> f (unsigned int a)
> {
>    int _1;
>    int _2;
> 
>    <bb 2> [local count: 1073741824]:
>    _1 = __builtin_clz (a_4(D));
>    _2 = 32 - _1;
>    if (_2 != 0)
>      goto <bb 3>; [33.00%]
>    else
>      goto <bb 4>; [67.00%]
> 
>    <bb 3> [local count: 354334800]:
>    __printf_chk (1, "test");
> 
>    <bb 4> [local count: 1073741824]:
>    return _2;
> }
> 
> I tried to rebuild defining  CLZ_DEFINED_VALUE_AT_ZERO(MODE, VALUE)
> ((VALUE) = 32, 2) but this doesn't seem to have any effect.
I would look to see the first pass where the condition gets removed 
using -fdump-tree-all-details.  That should give you strong hints about 
what to look at next.

Conceptually something has determined that _2 never has the value 0 
which through backwards propagation would indicate that __builtin_clz 
never returns the value 32.

Jeff