cfgexpand: Update partition size when merging variables

cfgexpand sorts variables by decreasing size, so when merging a later
variable into an earlier one, there's usually no need to update the
merged size.

But for poly_int sizes, the sort function just uses a lexicographical
comparison of the coefficients, so e.g. 2X+2 comes before 0X+32.
Which is bigger depends on the runtime value of X.

This patch therefore takes the upper bound of the two sizes, which
is conservatively correct for variable-length vectors and a no-op
on other targets.

It's probably a bad idea to merge fixed-length and variable-length
variables in practice, but that's really an optimisation decision.
I think we should have this patch as a correctness fix either way.

This is easiest to test using the ACLE, but in principle it could happen
for autovectorised code too, e.g. when using OpenMP vector variables.
It's therefore a regression from GCC 8.

2020-01-22  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* cfgexpand.c (union_stack_vars): Update the size.

gcc/testsuite/
	* gcc.target/aarch64/sve/acle/general/stack_vars_1.c: New test.

1 files changed, 3 insertions, 0 deletions

diff --git a/gcc/cfgexpand.c b/gcc/cfgexpand.c
index fb0575b146e..9864e4344d2 100644
--- a/gcc/cfgexpand.c
+++ b/gcc/cfgexpand.c
@@ -862,6 +862,9 @@ union_stack_vars (size_t a, size_t b)
   stack_vars[b].representative = a;
   stack_vars[a].next = b;
 
+  /* Make sure A is big enough to hold B.  */
+  stack_vars[a].size = upper_bound (stack_vars[a].size, stack_vars[b].size);
+
   /* Update the required alignment of partition A to account for B.  */
   if (stack_vars[a].alignb < stack_vars[b].alignb)
     stack_vars[a].alignb = stack_vars[b].alignb;