fold-const: Fold larger VIEW_CONVERT_EXPRs [PR113462]

fold-const: Fold larger VIEW_CONVERT_EXPRs [PR113462]

On Mon, Jan 22, 2024 at 11:27:52AM +0100, Richard Biener wrote:
> We run into
>
> static tree
> native_interpret_int (tree type, const unsigned char *ptr, int len)
> {
> ...
>   if (total_bytes > len
>       || total_bytes * BITS_PER_UNIT > HOST_BITS_PER_DOUBLE_INT)
>     return NULL_TREE;
>
> OTOH using a V_C_E to "truncate" a _BitInt looks wrong?  OTOH the
> check doesn't really handle native_encode_expr using the "proper"
> wide_int encoding however that's exactly handled.  So it might be
> a pre-existing issue that's only uncovered by large _BitInts
> (__int128 might show similar issues?)

I guess the || total_bytes * BITS_PER_UNIT > HOST_BITS_PER_DOUBLE_INT
conditions make no sense, all we care is whether it fits in the buffer
or not.
But then there is
fold_view_convert_expr
(and other spots) which use
  /* We support up to 1024-bit values (for GCN/RISC-V V128QImode).  */
  unsigned char buffer[128];
or something similar.
This patch fixes even that by using a XALLOCAVEC allocated buffer
if the type size is 129 .. 8192 bytes.

2024-01-22  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/113462
	* fold-const.cc (native_interpret_int): Don't punt if total_bytes
	is larger than HOST_BITS_PER_DOUBLE_INT / BITS_PER_UNIT.
	(fold_view_convert_expr): Use XALLOCAVEC buffers for types with
	sizes between 129 and 8192 bytes.