[PATCH] tree-optimization/127377 - wrong pruning of non-constant partial defs

[Richard Biener]

The following fixes the failure to adjust for sub-byte padding of
the partial definition before the object when pruning the size
to sub-byte padding after the object.

Bootstrapped and tested on x86_64-unknown-linux-gnu, pushed.

        PR tree-optimization/127377
        * tree-ssa-sccvn.cc (vn_walk_cb_data::push_partial_def):
        Properly consider the padding before the object when
        pruning the size.

        * gcc.dg/torture/pr127377.c: New testcase.
---
 gcc/testsuite/gcc.dg/torture/pr127377.c | 40 +++++++++++++++++++++++++
 gcc/tree-ssa-sccvn.cc                   |  5 ++--
 2 files changed, 43 insertions(+), 2 deletions(-)
 create mode 100644 gcc/testsuite/gcc.dg/torture/pr127377.c

diff --git a/gcc/testsuite/gcc.dg/torture/pr127377.c 
b/gcc/testsuite/gcc.dg/torture/pr127377.c
new file mode 100644
index 00000000000..13fb8f25ca7
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/torture/pr127377.c
@@ -0,0 +1,40 @@
+/* { dg-do run } */
+/* { dg-require-effective-target int32plus } */
+
+__attribute__((noipa))
+int q(void) {
+  struct h {
+    unsigned i : 1;
+    int : 1;
+    unsigned j : 3;
+    int k : 7;
+    int : 3;
+    int aa : 6;
+    unsigned l : 7;
+  };
+  union {
+    int m;
+    signed char b[4];
+    struct h ab;
+  } n = {};
+  int o;
+  n.m = 0x550f;
+  for (int i = 0; i < 1; i++) 
+  {
+    n.b[2] = n.ab.i + n.ab.aa;
+    n.m = n.m & ~28u | ((0x7 ^ n.ab.aa) & 7) << 2;
+  }
+  n.b[3] = 0;
+  n.m = n.m | 0x08000000;
+  n.b[2] = 0x7;
+  o = n.ab.l;
+  return o;
+}
+
+int main()
+{
+  int t1 =  q();
+  if (t1 != 64)
+    __builtin_abort ();
+  return 0;
+}
diff --git a/gcc/tree-ssa-sccvn.cc b/gcc/tree-ssa-sccvn.cc
index bfc227f7ca0..997a37a6647 100644
--- a/gcc/tree-ssa-sccvn.cc
+++ b/gcc/tree-ssa-sccvn.cc
@@ -2080,8 +2080,9 @@ vn_walk_cb_data::push_partial_def (pd_data pd,
          pd.size -= o;
          pd.offset += o;
        }
-      if (pd.size > maxsizei)
-       pd.size = maxsizei + ((pd.size - maxsizei) % BITS_PER_UNIT);
+      if (pd.size + pd.offset > offseti + maxsizei)
+       pd.size = maxsizei + ((pd.size + pd.offset - offseti - maxsizei)
+                             % BITS_PER_UNIT);
     }
 
   pd.offset -= offseti;
-- 
2.51.0