[Bug tree-optimization/115073] RISC-V: Gimple fold not honor C[LT]Z_DEFINED_VALUE_AT_ZERO

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115073

--- Comment #4 from Andrew Pinski  ---
Note -fsanitize=undefined does detect this undefinedness at runtime already
too.

[Bug tree-optimization/115073] RISC-V: Gimple fold not honor C[LT]Z_DEFINED_VALUE_AT_ZERO

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115073

--- Comment #3 from Jakub Jelinek  ---
Or another option is to use C23 , that is well defined for all
values.
But you need recent glibc (2.39 snapshots or 2.40) & gcc (14) for that.

[Bug tree-optimization/115073] RISC-V: Gimple fold not honor C[LT]Z_DEFINED_VALUE_AT_ZERO

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115073

Andrew Pinski  changed:

   What|Removed |Added

 Resolution|--- |INVALID
 Status|UNCONFIRMED |RESOLVED

--- Comment #2 from Andrew Pinski  ---
Invalid as explained.  The builtin has undefined behavior no matter what the
backend says is the value at 0.

[Bug tree-optimization/115073] RISC-V: Gimple fold not honor C[LT]Z_DEFINED_VALUE_AT_ZERO

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115073

Jakub Jelinek  changed:

   What|Removed |Added

 CC||jakub at gcc dot gnu.org

--- Comment #1 from Jakub Jelinek  ---
That is misunderstanding of what C[LT]Z_DEFINED_VALUE_AT_ZERO is.
The builtin has always UB on zero.
If you want to define it for zero, use x ? __builtin_clz (x) : 32 (and the
like) or
__builtin_clzg (x, 32) in the source.
C[LT]Z_DEFINED_VALUE_AT_ZERO returning 2 then makes sure the compiler can fold
those conditionals or builtins into .CLZ internal calls with 2 arguments which
makes it well defined for all inputs.