Hi!

In the following testcase we try to std::bit_cast a (pair of) integral
value(s) which has some non-zero bits in the place of x86 long double
(for 64-bit 16 byte type with 10 bytes actually loaded/stored by hw,
for 32-bit 12 byte) and starting with my PR104522 change we reject that
as native_interpret_expr fails on it.  The PR104522 change extends what
has been done before for MODE_COMPOSITE_P (but those don't have any padding
bits) to all floating point types, because e.g. the exact x86 long double
has various bit combinations we don't support, like
pseudo-(denormals,infinities,NaNs) or unnormals.  The HW handles some of
those as exceptional cases and others similarly to the non-pseudo ones.
But for the padding bits it actually doesn't load/store those bits at all,
it loads/stores 10 bytes.  So, I think we should exempt the padding bits
from the reverse comparison (the native_encode_expr bits for the padding
will be all zeros), which the following patch does.  For bit_cast it is
similar to e.g. ignoring padding bits if the destination is a structure
which has padding bits in there.

The change changed auto-init-4.c to how it has been behaving before the
PR105259 change, where some more VCEs can be now done.

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

2023-03-02  Jakub Jelinek  <ja...@redhat.com>

        PR c++/108934
        * fold-const.cc (native_interpret_expr) <case REAL_CST>: Before memcmp
        comparison copy the bytes from ptr to a temporary buffer and clearing
        padding bits in there.

        * gcc.target/i386/auto-init-4.c: Revert PR105259 change.
        * g++.target/i386/pr108934.C: New test.

--- gcc/fold-const.cc.jj        2023-01-04 10:52:43.124897826 +0100
+++ gcc/fold-const.cc   2023-03-01 16:49:14.531490482 +0100
@@ -8873,11 +8873,13 @@ native_interpret_expr (tree type, const
             valid values that GCC can't really represent accurately.
             See PR95450.  Even for other modes, e.g. x86 XFmode can have some
             bit combinationations which GCC doesn't preserve.  */
-         unsigned char buf[24];
+         unsigned char buf[24 * 2];
          scalar_float_mode mode = SCALAR_FLOAT_TYPE_MODE (type);
          int total_bytes = GET_MODE_SIZE (mode);
+         memcpy (buf + 24, ptr, total_bytes);
+         clear_type_padding_in_mask (type, buf + 24);
          if (native_encode_expr (ret, buf, total_bytes, 0) != total_bytes
-             || memcmp (ptr, buf, total_bytes) != 0)
+             || memcmp (buf + 24, buf, total_bytes) != 0)
            return NULL_TREE;
          return ret;
        }
--- gcc/testsuite/gcc.target/i386/auto-init-4.c.jj      2022-04-13 
15:42:39.105365390 +0200
+++ gcc/testsuite/gcc.target/i386/auto-init-4.c 2023-03-02 08:56:53.788029181 
+0100
@@ -15,6 +15,5 @@ long double foo()
 }
 
 
-/* The long double init isn't expanded optimally, see PR105259.  For ia32
-   it uses zero-initialization.  */
-/* { dg-final { scan-assembler-times "long\t-16843010" 3 } } */
+/* { dg-final { scan-assembler-times "long\t-16843010" 5  { target { ! ia32 } 
} } } */
+/* { dg-final { scan-assembler-times "long\t-16843010" 3  { target { ia32 } } 
} } */
--- gcc/testsuite/g++.target/i386/pr108934.C.jj 2023-03-01 17:04:19.931299866 
+0100
+++ gcc/testsuite/g++.target/i386/pr108934.C    2023-03-01 17:03:27.567062785 
+0100
@@ -0,0 +1,28 @@
+// PR c++/108934
+// { dg-do compile { target c++11 } }
+
+struct S { unsigned long long a[2]; };
+struct T { unsigned long long b[6]; };
+struct U { unsigned long long c[2]; long double d; unsigned long long e[2]; };
+
+#if __SIZEOF_LONG_DOUBLE__ == 16 && __LDBL_MANT_DIG__ == 64 && 
__SIZEOF_LONG_LONG__ == 8
+constexpr long double
+foo (S x)
+{
+  return __builtin_bit_cast (long double, x);
+}
+
+constexpr S a = { 0ULL, 0xffffffffffff0000ULL };
+constexpr long double b = foo (a);
+static_assert (b == 0.0L, "");
+
+constexpr U
+bar (T x)
+{
+  return __builtin_bit_cast (U, x);
+}
+
+constexpr T c = { 0ULL, 0ULL, 0ULL, 0xffffffffffff0000ULL, 0ULL, 0ULL };
+constexpr U d = bar (c);
+static_assert (d.d == 0.0L, "");
+#endif

        Jakub

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