[Bug tree-optimization/108341] argument to `__builtin_ctz` should be assumed non-zero when CTZ_DEFINED_VALUE_AT_ZERO says it is undefined

[lh_mouse at 126 dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108341

--- Comment #7 from LIU Hao  ---
(In reply to Jakub Jelinek from comment #4)
> I think 0 argument for __builtin_c[lt]z{,l,ll,imax} is always undefined, 0
> argument
> to .C[LT]Z (internal calls) is undefined if C[LT]Z_DEFINED_VALUE_AT_ZERO is
> not 2 and
> 0 argument to C[LT]Z RTL is undefined if C[LT]Z_DEFINED_VALUE_AT_ZERO is not
> non-zero.

I agree with this.


#94801 mentioned the `if(value == 0) __builtin_unreachable();` trick, but it
isn't an option if the argument is really possibly a zero:

(https://gcc.godbolt.org/z/dePvcMhTr)
```
#include 

uint32_t
my_tzcnt(uint32_t value)
  {
return (value == 0) ? 32 : __builtin_ctz(value);
  }
```

This can be TZCNT if the CPU supports it.

[Bug tree-optimization/108341] argument to `__builtin_ctz` should be assumed non-zero when CTZ_DEFINED_VALUE_AT_ZERO says it is undefined

[aldyh at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108341

Aldy Hernandez  changed:

   What|Removed |Added

   Severity|normal  |enhancement

--- Comment #6 from Aldy Hernandez  ---
Huh.  Didn't know you could do that.  Thanks.

FWIW, the function is actually:

gimple_infer_range::gimple_infer_range (gimple *s)

[Bug tree-optimization/108341] argument to `__builtin_ctz` should be assumed non-zero when CTZ_DEFINED_VALUE_AT_ZERO says it is undefined

[amacleod at redhat dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108341

--- Comment #5 from Andrew Macleod  ---
(In reply to Aldy Hernandez from comment #2)
> (In reply to Martin Liška from comment #1)
> > May be an opportunity for Ranger?
> 
> Hmmm... I don't think so:
> 
>  :
> value.0_1 = (unsigned int) value_4(D);
> _2 = __builtin_ctz (value.0_1);
> r = _2;
> _3 = value_4(D) != 0;
> _7 = (int) _3;
> return _7;
> 
> We could add an op1_range operator to class cfn_clz to return nonzero for
> op1, but that would only work if we knew _2 to be anything...and have no
> info on _2.

Seems more like a candidate for gimpe_infer::gimple_infer (gimple *s).

THe side effect to register would be to check if 's' is a builtin_ctz and if
so, call add_nonzero (operand1) if whatever those other conditions are are
matched which make it true.

That should register a non-zero inferred range on value.0_1 after the
assignment of _2.


=== BB 2 
Partial equiv (value.0_1 pe32 value_4(D))
 :
value.0_1 = (unsigned int) value_4(D);
_2 = __builtin_ctz (value.0_1);
r = _2;
_3 = value_4(D) != 0;
_7 = (int) _3;
return _7;

I see ranger also registers a 32 bit equivalence between value.0_1 and value_4,
so in theory we would then be able to determine that value_4 is also non-zero
for the comparison.

[Bug tree-optimization/108341] argument to `__builtin_ctz` should be assumed non-zero when CTZ_DEFINED_VALUE_AT_ZERO says it is undefined

[jakub at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108341

Jakub Jelinek  changed:

   What|Removed |Added

 CC||jakub at gcc dot gnu.org

--- Comment #4 from Jakub Jelinek  ---
I think 0 argument for __builtin_c[lt]z{,l,ll,imax} is always undefined, 0
argument
to .C[LT]Z (internal calls) is undefined if C[LT]Z_DEFINED_VALUE_AT_ZERO is not
2 and
0 argument to C[LT]Z RTL is undefined if C[LT]Z_DEFINED_VALUE_AT_ZERO is not
non-zero.