Fix PR rtl-optimization/102306

This is a duplication of volatile loads introduced during GCC 9 development
by the 2->2 mechanism of the RTL combiner.  There is already a substantial
checking for volatile references in can_combine_p but it implicitly assumes
that the combination reduces the number of instructions, which is of course
not the case here.  So the fix teaches try_combine to abort the combination
when it is about to make a copy of volatile references to preserve them.

gcc/
	PR rtl-optimization/102306
	* combine.c (try_combine): Abort the combination if we are about to
	duplicate volatile references.

gcc/testsuite/
	* gcc.target/sparc/20210917-1.c: New test.

2 files changed, 29 insertions, 0 deletions

diff --git a/gcc/combine.c b/gcc/combine.c
index 290a366..892c834 100644
--- a/gcc/combine.c
+++ b/gcc/combine.c
@@ -3063,6 +3063,16 @@ try_combine (rtx_insn *i3, rtx_insn *i2, rtx_insn *i1, rtx_insn *i0,
       return 0;
     }
 
+  /* We cannot safely duplicate volatile references in any case.  */
+
+  if ((added_sets_2 && volatile_refs_p (PATTERN (i2)))
+      || (added_sets_1 && volatile_refs_p (PATTERN (i1)))
+      || (added_sets_0 && volatile_refs_p (PATTERN (i0))))
+    {
+      undo_all ();
+      return 0;
+    }
+
   /* Count how many auto_inc expressions there were in the original insns;
      we need to have the same number in the resulting patterns.  */
 
diff --git a/gcc/testsuite/gcc.target/sparc/20210917-1.c b/gcc/testsuite/gcc.target/sparc/20210917-1.c
new file mode 100644
index 0000000..03e8bc5
--- /dev/null
+++ b/gcc/testsuite/gcc.target/sparc/20210917-1.c
@@ -0,0 +1,19 @@
+/* PR rtl-optimization/102306 */
+/* Reported by Daniel Cederman <cederman@gaisler.com> */
+
+/* { dg-do compile } */
+/* { dg-require-effective-target ilp32 } */
+/* { dg-options "-O -mcpu=v8" } */
+
+extern void foo (void);
+
+void test (volatile unsigned char *a) 
+{ 
+  char b = *a;
+  if (!b)
+    return;
+  if (b & 2)
+    foo ();
+}
+
+/* { dg-final { scan-assembler-times "ldub" 1 } } */