middle-end/111821 - compile-time/memory-hog with large copy

The following fixes a compile-time/memory-hog when performing a
large aggregate copy to a small object allocated to a register.
While store_bit_field_1 called by store_integral_bit_field will
do nothign for accesses outside of the target the loop over the
source in store_integral_bit_field will still code-generate
the read parts for all words in the source.  The following copies
the noop condition from store_bit_field_1 and terminates the
loop when it runs forward or avoid code-generating the read parts
when not.

	PR middle-end/111821
	* expmed.cc (store_integral_bit_field): Terminate the
	word-wise copy loop when we get out of the destination
	and do a forward copy.  Skip the word if it would be
	outside of the destination in case of a backward copy.

	* gcc.dg/torture/pr111821.c: New testcase.

2 files changed, 21 insertions, 0 deletions

diff --git a/gcc/expmed.cc b/gcc/expmed.cc
index 154964b..2ca93e3 100644
--- a/gcc/expmed.cc
+++ b/gcc/expmed.cc
@@ -986,6 +986,18 @@ store_integral_bit_field (rtx op0, opt_scalar_int_mode op0_mode,
 	    = backwards ^ reverse
 	      ? MAX ((int) bitsize - (i + 1) * BITS_PER_WORD, 0)
 	      : i * BITS_PER_WORD;
+
+	  /* No further action is needed if the target is a register and if
+	     this field lies completely outside that register.  */
+	  if (REG_P (op0) && known_ge (bitnum + bit_offset,
+				       GET_MODE_BITSIZE (GET_MODE (op0))))
+	    {
+	      if (backwards ^ reverse)
+		continue;
+	      /* For forward operation we are finished.  */
+	      return true;
+	    }
+
 	  /* Starting word number in the value.  */
 	  const unsigned int wordnum
 	    = backwards
diff --git a/gcc/testsuite/gcc.dg/torture/pr111821.c b/gcc/testsuite/gcc.dg/torture/pr111821.c
new file mode 100644
index 0000000..eec6eed
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/torture/pr111821.c
@@ -0,0 +1,9 @@
+/* { dg-do compile } */
+
+typedef union { char a[__INT_MAX__ / 4]; } T;
+unsigned short f(const void *p)
+{
+  unsigned short v;
+  *(T *)(void *)(&v) = *(const T *)p;
+  return v;
+}