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.

4 files changed, 43 insertions, 0 deletions

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index 2f229b9..272968a 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,7 @@
+2020-01-22  Richard Sandiford  <richard.sandiford@arm.com>
+
+	* cfgexpand.c (union_stack_vars): Update the size.
+
 2020-01-22  Richard Biener  <rguenther@suse.de>
 
 	PR tree-optimization/93381
diff --git a/gcc/cfgexpand.c b/gcc/cfgexpand.c
index fb0575b1..9864e43 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;
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index 3bd6407..d22747b 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,5 +1,9 @@
 2020-01-22  Richard Sandiford  <richard.sandiford@arm.com>
 
+	* gcc.target/aarch64/sve/acle/general/stack_vars_1.c: New test.
+
+2020-01-22  Richard Sandiford  <richard.sandiford@arm.com>
+
 	* gcc.target/aarch64/sve/tls_preserve_1.c: Require tls_native.
 	* gcc.target/aarch64/sve/tls_preserve_2.c: Likewise.
 	* gcc.target/aarch64/sve/tls_preserve_3.c: Likewise.
diff --git a/gcc/testsuite/gcc.target/aarch64/sve/acle/general/stack_vars_1.c b/gcc/testsuite/gcc.target/aarch64/sve/acle/general/stack_vars_1.c
new file mode 100644
index 0000000..860fa67f
--- /dev/null
+++ b/gcc/testsuite/gcc.target/aarch64/sve/acle/general/stack_vars_1.c
@@ -0,0 +1,32 @@
+/* { dg-do run { target aarch64_sve_hw } } */
+/* { dg-additional-options "-O2" } */
+
+#include <arm_sve.h>
+
+struct s { int x[32]; };
+
+void __attribute__((noipa)) consume (void *ptr) {}
+
+void __attribute__((noipa))
+check_var (svint32_t *ptr)
+{
+  svbool_t pg = svptrue_b8 ();
+  if (svptest_any (pg, svcmpne (pg, *ptr, svindex_s32 (0, 1))))
+    __builtin_abort ();
+}
+
+int
+main (void)
+{
+  svint32_t res = svindex_s32 (0, 1);
+  {
+    __SVBool_t pg = svptrue_b8 ();
+    consume (&pg);
+  }
+  {
+    struct s zeros = { 0 };
+    consume (&zeros);
+  }
+  check_var (&res);
+  return 0;
+}