expander: Optimize store_expr from STRING_CST [PR95052]

expander: Optimize store_expr from STRING_CST [PR95052]

In the following testcase, store_expr of e.g. 97 bytes long string literal
into 1MB long array is implemented by copying the 97 bytes from .rodata
section, followed by clearing the remaining bytes.  But, as the STRING_CST
has type char[1024*1024], we actually allocate whole 1MB in .rodata section
for it, even when we only use the first 97 bytes from that.

The following patch tweaks it so that if we are going to initialize only the
small part from it, we don't emit all the zeros that we never use after it.

2020-05-29  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/95052
	* expr.c (store_expr): If expr_size is constant and significantly
	larger than TREE_STRING_LENGTH, set temp to just the
	TREE_STRING_LENGTH portion of the STRING_CST.

	* gcc.target/i386/pr95052.c: New test.