[gcc r10-10134] Fix PR rtl-optimization/102306

[Eric Botcazou]

https://gcc.gnu.org/g:0c1af84f181237b8a90ae391152dbdcb9c6ab064

commit r10-10134-g0c1af84f181237b8a90ae391152dbdcb9c6ab064
Author: Eric Botcazou <ebotcazou@adacore.com>
Date:   Fri Sep 17 10:12:12 2021 +0200

    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.

Diff:
---
 gcc/combine.c                               | 10 ++++++++++
 gcc/testsuite/gcc.target/sparc/20210917-1.c | 19 +++++++++++++++++++
 2 files changed, 29 insertions(+)

diff --git a/gcc/combine.c b/gcc/combine.c
index 4584b2e74ae..d7135f393de 100644
--- a/gcc/combine.c
+++ b/gcc/combine.c
@@ -3214,6 +3214,16 @@ 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 00000000000..03e8bc58db9
--- /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 } } */