On Tue, Feb 16, 2021 at 08:34:41PM -0700, Martin Sebor via Gcc-patches wrote:
> + if (integer_all_onesp (nelts))
> + /* Zero length array. */
> + eltsize = 0;
> + else
> {
> - tree bnds[] = { TYPE_MIN_VALUE (dom), TYPE_MAX_VALUE (dom) };
> - if (TREE_CODE (arg) == COMPONENT_REF)
> - {
> - offset_int size = maxobjsize;
> - if (tree fldsize = component_ref_size (arg))
> - size = wi::to_offset (fldsize);
> - arrbounds[1] = wi::lrshift (size, eltsizelog2);
> - }
> - else if (array_at_struct_end_p (arg) || !bnds[0] || !bnds[1])
> - arrbounds[1] = wi::lrshift (maxobjsize, eltsizelog2);
> - else
> - arrbounds[1] = (wi::to_offset (bnds[1]) - wi::to_offset
> (bnds[0])
> - + 1) * eltsize;
> + tree esz = TYPE_SIZE_UNIT (TREE_TYPE (reftype));
> + if (TREE_CODE (esz) == INTEGER_CST)
> + /* Array element is not a VLA. */
> + eltsize = wi::to_offset (esz);
> }
> +
> + if (!array_at_struct_end_p (arg)
> + && TREE_CODE (nelts) == INTEGER_CST)
> + arrbounds[1] = (wi::to_offset (nelts) + 1) * eltsize;
> else
> - arrbounds[1] = wi::lrshift (maxobjsize, eltsizelog2);
> + {
> + /* Use log2 of size to convert the array byte size in to its
> + upper bound in elements. */
> + const offset_int eltsizelog2 = wi::floor_log2 (eltsize);
> + arrbounds[1] = wi::lrshift (maxobjsize, eltsizelog2);
So, what will this do for zero length arrays at the end of struct?
eltsize will be 0 and wi::floor_log2 (eltsize) is -1, and shifting by -1,
while maybe not UB in wide_int computations, is certainly just weird.
Why do you use eltsize = 0 for the [0] arrays? They can still have their
element size and if array_at_struct_end_p and the element type is not a variable
length type, using the actual eltsize seems better.
Only when !array_at_struct_end_p we should ensure arrbounds[1] will be 0
and indeed that (wi::to_offset (nelts) + 1) * eltsize would likely not do
that because wi::to_offset is -1ULL or so, not -1.
Also, I'm not sure I understand the right shift by floor_log2 of eltsize,
why can't you simply divide maxobjsize by eltsize (if eltsize is not 0).
Jakub
