On 07/01/2011 01:58 PM, Stubbs, Andrew wrote:
Given this test case:unsigned long long foo (unsigned long long a, signed char *b, signed char *c) { return a + *b * *c; } Those rules say that it should not be suitable for optimization because there's an implicit cast from signed int to unsigned long long.
Got it now! Casts from signed to unsigned are not value-preserving, but they are "bit-preserving": s32->s64 obviously is, and s32->u64 has the same result bit-by-bit as the s64 result. The fact that s64 has an implicit 1111... in front, while an u64 has an implicit 0000... does not matter.
Is this the meaning of the predicate you want? I think so, based on the discussion, but it's hard to say without seeing the cases enumerated (i.e. a patch).
However, perhaps there is a catch. We can do the following thought experiment. What would happen if you had multiple widening multiplies? Like 8-bit signed to 64-bit unsigned and then 64-bit unsigned to 128-bit unsigned? I believe in this case you couldn't optimize 8-bit signed to 128-bit unsigned. Would your code do it?
Paolo
