On 2/11/22 06:26, Richard Biener wrote:
The following attempts to address gimplification of
    ... = VIEW_CONVERT_EXPR<int[4]>((i & 1) != 0 ? inv : src)[i];

which is problematic since gimplifying the base object
? inv : src produces a register temporary but GIMPLE does not
really support a register as a base for an ARRAY_REF (even
though that's not strictly validated it seems as can be seen
at -O0).
I suppose that isn't easy to fix?

And COMPONENT_REF has the same problem?

Interestingly the C++ frontend avoids this issue
by emitting the following GENERIC instead:

    ... = (i & 1) != 0 ? VIEW_CONVERT_EXPR<int[4]>(inv)[i] : 
VIEW_CONVERT_EXPR<int[4]>(src)[i];
Yes, because in C++ ?: of two lvalues is an lvalue.

The proposed patch below fixes things up when using an rvalue
as the base is OK by emitting a copy from a register base to a
non-register one.  The ?: as lvalue extension seems to be gone
for C, C++ again unwraps the COND_EXPR in that case.

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

OK?
OK, assuming "yes" answers to my questions above.

Thanks,
Richard.

2022-02-11  Richard Biener  <rguent...@suse.de>

        PR middle-end/104497
        * gimplify.cc (gimplify_compound_lval): Make sure the
        base is a non-register if needed and possible.

        * c-c++-common/torture/pr104497.c: New testcase.
---
  gcc/gimplify.cc                               | 17 ++++++++++++++---
  gcc/testsuite/c-c++-common/torture/pr104497.c | 12 ++++++++++++
  2 files changed, 26 insertions(+), 3 deletions(-)
  create mode 100644 gcc/testsuite/c-c++-common/torture/pr104497.c

diff --git a/gcc/gimplify.cc b/gcc/gimplify.cc
index 8d676fb96c8..cdf1ccbe48b 100644
--- a/gcc/gimplify.cc
+++ b/gcc/gimplify.cc
@@ -250,6 +250,7 @@ static enum gimplify_status gimplify_compound_expr (tree *, 
gimple_seq *, bool);
  static hash_map<tree, tree> *oacc_declare_returns;
  static enum gimplify_status gimplify_expr (tree *, gimple_seq *, gimple_seq *,
                                           bool (*) (tree), fallback_t, bool);
+static void prepare_gimple_addressable (tree *, gimple_seq *);
/* Shorter alias name for the above function for use in gimplify.cc
     only.  */
@@ -3126,10 +3127,12 @@ gimplify_compound_lval (tree *expr_p, gimple_seq 
*pre_p, gimple_seq *post_p,
       gimplified before gimplifying the size expressions.
So we do this in three steps. First we deal with variable
-     bounds, sizes, and positions, then we gimplify the base,
-     then we deal with the annotations for any variables in the
-     components and any indices, from left to right.  */
+     bounds, sizes, and positions, then we gimplify the base and
+     ensure it is memory if needed, then we deal with the annotations
+     for any variables in the components and any indices, from left
+     to right.  */
+ bool need_non_reg = false;
    for (i = expr_stack.length () - 1; i >= 0; i--)
      {
        tree t = expr_stack[i];
@@ -3165,6 +3168,7 @@ gimplify_compound_lval (tree *expr_p, gimple_seq *pre_p, 
gimple_seq *post_p,
                  TREE_OPERAND (t, 3) = elmt_size;
                }
            }
+         need_non_reg = true;
        }
        else if (TREE_CODE (t) == COMPONENT_REF)
        {
@@ -3186,6 +3190,7 @@ gimplify_compound_lval (tree *expr_p, gimple_seq *pre_p, 
gimple_seq *post_p,
                  TREE_OPERAND (t, 2) = offset;
                }
            }
+         need_non_reg = true;
        }
      }
@@ -3196,6 +3201,12 @@ gimplify_compound_lval (tree *expr_p, gimple_seq *pre_p, gimple_seq *post_p,
                        fallback | fb_lvalue);
    ret = MIN (ret, tret);
+ /* Step 2a: if we have component references we do not support on
+     registers then make sure the base isn't a register.  Of course
+     we can only do so if an rvalue is OK.  */
+  if (need_non_reg && (fallback & fb_rvalue))
+    prepare_gimple_addressable (p, pre_p);
+
    /* Step 3: gimplify size expressions and the indices and operands of
       ARRAY_REF.  During this loop we also remove any useless conversions.  */
diff --git a/gcc/testsuite/c-c++-common/torture/pr104497.c b/gcc/testsuite/c-c++-common/torture/pr104497.c
new file mode 100644
index 00000000000..c63fc021e03
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/torture/pr104497.c
@@ -0,0 +1,12 @@
+/* { dg-do compile } */
+
+typedef int __attribute__((vector_size(16))) vec_t;
+
+vec_t src, inv, res;
+
+void test(int i)
+{
+    vec_t y={0};
+    y[i] = (i & 1 ? inv : src)[i];
+    res = y;
+}


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