[committed] bswap: Fix up a thinko with empty CONSTRUCTORs [PR98378]

[Jakub Jelinek]

Hi!

The code I've added recently in find_bswap_or_nop for VECTOR CONSTRUCTORs
is missing punt on an important case - namely empty CONSTRUCTORs, because in that
case the loop will not initialize *n and the code after the loop will then
use the uninitialized structure.

Fixed thusly, bootstrapped/regtested on x86_64-linux and i686-linux,
committed to trunk.

No added testcase, as it exhibits only under valgrind, and on existing
testcases already.

2020-12-19  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/98378
	* gimple-ssa-store-merging.c (find_bswap_or_nop): Punt if CONSTRUCTOR
	has no elements.

--- gcc/gimple-ssa-store-merging.c.jj	2020-12-16 16:14:44.000000000 +0100
+++ gcc/gimple-ssa-store-merging.c	2020-12-18 23:12:59.671282002 +0100
@@ -873,6 +873,8 @@ find_bswap_or_nop (gimple *stmt, struct
       if (sz != 16 && sz != 32 && sz != 64)
 	return NULL;
       tree rhs = gimple_assign_rhs1 (stmt);
+      if (CONSTRUCTOR_NELTS (rhs) == 0)
+	return NULL;
       tree eltype = TREE_TYPE (TREE_TYPE (rhs));
       unsigned HOST_WIDE_INT eltsz
 	= int_size_in_bytes (eltype) * BITS_PER_UNIT;

	Jakub