On 1/5/19 1:31 PM, Jakub Jelinek wrote:
> On Sat, Jan 05, 2019 at 12:20:24PM +0000, Bernd Edlinger wrote:
>> co-incidentally my "Make strlen range computations more conservative" patch
>> contained a fix on the same spot, I just did not have a test case for it:
>>
>> @@ -3184,7 +3146,10 @@ get_min_string_length (tree rhs, bool *f
>> && TREE_READONLY (rhs))
>> rhs = DECL_INITIAL (rhs);
>>
>> - if (rhs && TREE_CODE (rhs) == STRING_CST)
>> + if (rhs && TREE_CODE (rhs) == STRING_CST
>> + && tree_to_uhwi (TYPE_SIZE_UNIT (TREE_TYPE (TREE_TYPE (rhs)))) == 1
>> + && TREE_STRING_LENGTH (rhs) > 0
>> + && TREE_STRING_POINTER (rhs) [TREE_STRING_LENGTH (rhs) - 1] == '\0')
>> {
>> *full_string_p = true;
>> return strlen (TREE_STRING_POINTER (rhs));
>>
>>
>> additionally to your patch this tests the string is in fact a single-byte
>> string.
>> since strlen returns garbage otherwise.
>
> Multi-byte STRING_CSTs seem to be stored in the target byte order, e.g.
> L"abcde" in a x86_64-linux -> powerpc64-linux cross is still
> "\000\000\000a\000\000\000b\000\000\000c\000\000\000d\000\000\000\000"
> so I think strlen is exactly what we want. The tree-ssa-strlen.c pass
> doesn't track string lengths in whatever units it has, it tracks number of
> non-zero bytes followed by zero byte known at certain address, or, if there
> is no zero byte known, the minimum amount of non-zero bytes known at certain
> address.
>
Well, yes it works, but this can only optimize invalid code like
strlen((char*)L"wide").
However, optimizing invalid stuff away does both defeat warnings in later
passes,
and is also not helpful for debugging the resulting code in general.
> And, your patch has also the bad effect that it won't return any length for
> the strings that aren't zero terminated. We do want to return 9 for the
> "abcdefghi" string without zero terminator at the end, just need to set
> *full_string_p to false. And, for "abcd\000fghi" string without zero
> termination at the end, we do want to return 4 and set *full_string_p to
> true.
>
Right, I did not consider the nul in the middle.
Bernd.
