[gcc r10-10677] c++: Fix up constant expression __builtin_convertvector folding [PR104472]

[Jakub Jelinek <jakub@gcc.gnu.org>]

https://gcc.gnu.org/g:f755dd4bda6cb1ae5c6d8e9c3a285faea236d3c3

commit r10-10677-gf755dd4bda6cb1ae5c6d8e9c3a285faea236d3c3
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Fri Feb 11 13:52:44 2022 +0100

    c++: Fix up constant expression __builtin_convertvector folding [PR104472]
    
    The following testcase ICEs, because due to the -frounding-math
    fold_const_call fails, which is it returns NULL, and returning NULL from
    cxx_eval* is wrong, all the callers rely on them to either return folded
    value or original with *non_constant_p = true.
    
    The following patch does that, and additionally falls through into the
    default case where there is diagnostics for the !ctx->quiet case too.
    
    2022-02-11  Jakub Jelinek  <jakub@redhat.com>
    
            PR c++/104472
            * constexpr.c (cxx_eval_internal_function) <case IFN_VEC_CONVERT>:
            Only return fold_const_call result if it is non-NULL.  Otherwise
            fall through into the default: case to return t, set *non_constant_p
            and emit diagnostics if needed.
    
            * g++.dg/cpp0x/constexpr-104472.C: New test.
    
    (cherry picked from commit 84993d94e13ad2ab3aee151bb5a5e767cf75d51e)

Diff:
---
 gcc/cp/constexpr.c                            | 9 +++------
 gcc/testsuite/g++.dg/cpp0x/constexpr-104472.C | 9 +++++++++
 2 files changed, 12 insertions(+), 6 deletions(-)

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index cd8ef069dde..a72b3641e90 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -1687,13 +1687,10 @@ cxx_eval_internal_function (const constexpr_ctx *ctx, tree t,
 						 false, non_constant_p,
 						 overflow_p);
 	if (TREE_CODE (arg) == VECTOR_CST)
-	  return fold_const_call (CFN_VEC_CONVERT, TREE_TYPE (t), arg);
-	else
-	  {
-	    *non_constant_p = true;
-	    return t;
-	  }
+	  if (tree r = fold_const_call (CFN_VEC_CONVERT, TREE_TYPE (t), arg))
+	    return r;
       }
+      /* FALLTHRU */
 
     default:
       if (!ctx->quiet)
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-104472.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-104472.C
new file mode 100644
index 00000000000..c9c8bf414fe
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-104472.C
@@ -0,0 +1,9 @@
+// PR c++/104472
+// { dg-options "-O2 -frounding-math" }
+// { dg-add-options float16 }
+// { dg-require-effective-target float16 }
+
+typedef short __attribute__((__vector_size__ (16))) V;
+typedef _Float16 __attribute__((__vector_size__ (16))) F;
+
+V v = __builtin_convertvector (__builtin_convertvector ((V){5534}, F), V) < 8;