[gcc r12-8492] c++: Fix up ICE on __builtin_shufflevector constexpr evaluation [PR105871]

[gcc r12-8492] c++: Fix up ICE on __builtin_shufflevector constexpr evaluation [PR105871]

[Jakub Jelinek]

https://gcc.gnu.org/g:0ddeeb11e45b9b7e9ebc18292a42769304bf3e44

commit r12-8492-g0ddeeb11e45b9b7e9ebc18292a42769304bf3e44
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Thu Jun 9 17:42:31 2022 +0200

    c++: Fix up ICE on __builtin_shufflevector constexpr evaluation [PR105871]
    
    As the following testcase shows, BIT_FIELD_REF result doesn't have to have
    just integral type, it can also have vector type.  And in that case
    cxx_eval_bit_field_ref just ICEs on it because it is unprepared for that
    case, creates the initial value with build_int_cst (sure, that one could be
    easily replaced with build_zero_cst) and then expects it can through shifts,
    ands and ors come up with the final value, but that doesn't work for
    vectors.
    
    We already call fold_ternary if whole is a VECTOR_CST, this patch does the
    same if the result doesn't have integral type.  And, there is no guarantee
    fold_ternary will succeed and the callers certainly don't expect NULL
    being returned, so it also diagnoses those as non-constant and returns
    original t in that case.
    
    2022-06-09  Jakub Jelinek  <jakub@redhat.com>
    
            PR c++/105871
            * constexpr.cc (cxx_eval_bit_field_ref): For BIT_FIELD_REF with
            non-integral result type use fold_ternary too like for BIT_FIELD_REFs
            from VECTOR_CST.  If fold_ternary returns NULL, diagnose non-constant
            expression, set *non_constant_p and return t, instead of returning
            NULL.
    
            * g++.dg/pr105871.C: New test.
    
    (cherry picked from commit 4c334e0e4ff05d3a7ca9ebb832428c446cd0ae8d)

Diff:
---
 gcc/cp/constexpr.cc             | 13 ++++++++++---
 gcc/testsuite/g++.dg/pr105871.C | 12 ++++++++++++
 2 files changed, 22 insertions(+), 3 deletions(-)

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 1e0d13e6785..1c17f0d14b8 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -4181,9 +4181,16 @@ cxx_eval_bit_field_ref (const constexpr_ctx *ctx, tree t,
   if (*non_constant_p)
     return t;
 
-  if (TREE_CODE (whole) == VECTOR_CST)
-    return fold_ternary (BIT_FIELD_REF, TREE_TYPE (t), whole,
-			 TREE_OPERAND (t, 1), TREE_OPERAND (t, 2));
+  if (TREE_CODE (whole) == VECTOR_CST || !INTEGRAL_TYPE_P (TREE_TYPE (t)))
+    {
+      if (tree r = fold_ternary (BIT_FIELD_REF, TREE_TYPE (t), whole,
+				 TREE_OPERAND (t, 1), TREE_OPERAND (t, 2)))
+	return r;
+      if (!ctx->quiet)
+	error ("%qE is not a constant expression", orig_whole);
+      *non_constant_p = true;
+      return t;
+    }
 
   start = TREE_OPERAND (t, 2);
   istart = tree_to_shwi (start);
diff --git a/gcc/testsuite/g++.dg/pr105871.C b/gcc/testsuite/g++.dg/pr105871.C
new file mode 100644
index 00000000000..3623c1d1366
--- /dev/null
+++ b/gcc/testsuite/g++.dg/pr105871.C
@@ -0,0 +1,12 @@
+// PR c++/105871
+// { dg-do compile }
+// { dg-options "-Wno-psabi" }
+
+typedef __attribute__((__vector_size__ ( 1))) unsigned char U;
+typedef __attribute__((__vector_size__ (16))) unsigned char V;
+
+U
+foo (void)
+{
+  return __builtin_shufflevector ((U){}, (V){}, 0);
+}