[PATCH] gimple-fold: Don't optimize wierdo floating point value reads [PR95450]

[Jakub Jelinek <jakub@redhat.com>]

Hi!

My patch to introduce native_encode_initializer to fold_ctor_reference
apparently broke gnulib/m4 on powerpc64.
There it uses a const union with two doubles and corresponding IBM double
double long double which actually is the largest normalizable long double
value (1 ulp higher than __LDBL_MAX__).  The reason our __LDBL_MAX__ is
smaller is that we internally treat the double double type as one having
106-bit precision, but it actually has a variable 53-bit to 2000-ish bit precision
and for the
0x1.fffffffffffff7ffffffffffffc000p+1023L
value gnulib uses we need 107-bit precision, therefore for GCC __LDBL_MAX__
is
0x1.fffffffffffff7ffffffffffff8000p+1023L
Before my changes, we wouldn't be able to fold_ctor_reference it and it
worked fine at runtime, but with the change we are able to do that, but
because it is larger than anything we can handle internally, we treat it
weirdly.  Similar problem would be if somebody creates this way valid,
but much more than 106 bit precision e.g. 1.0 + 1.0e-768.
Now, I think similar problem could happen e.g. on i?86/x86_64 with long
double there, it also has some weird values in the format, e.g. the
unnormals, pseudo infinities and various other magic values.

This patch for floating point types (including vector and complex types
with such elements) will try to encode the returned value again and punt
if it has different memory representation from the original.  Note, this
is only done in the path where native_encode_initializer was used, in order
not to affect e.g. just reading an unpunned long double value; the value
should be compiler generated in that case and thus should be properly
representable.  It will punt also if e.g. the padding bits are initialized
to non-zero values.

Or should I do this in native_interpret_real instead, so that we punt even
on say VIEW_CONVERT_EXPR from an integral value containing such weird bits?

And, do we want to do it for all floating point constants, or just
COMPOSITE_MODE_P (element_mode (type)) ones (i.e. only for double double)?

Bootstrapped/regtested on {x86_64,i686,powerpc64{,le}}-linux.

2020-08-12  Jakub Jelinek  <jakub@redhat.com>

	PR target/95450
	* gimple-fold.c (fold_ctor_reference): When interpreting bytes
	from native_encode_initializer into a floating point type,
	verify if it will be encoded back into the same memory representation
	and punt otherwise.

--- gcc/gimple-fold.c.jj	2020-08-04 11:31:26.580268603 +0200
+++ gcc/gimple-fold.c	2020-08-11 19:00:59.147564022 +0200
@@ -7090,7 +7090,19 @@ fold_ctor_reference (tree type, tree cto
 	  int len = native_encode_initializer (ctor, buf, size / BITS_PER_UNIT,
 					       offset / BITS_PER_UNIT);
 	  if (len > 0)
-	    return native_interpret_expr (type, buf, len);
+	    {
+	      ret = native_interpret_expr (type, buf, len);
+	      if (ret && FLOAT_TYPE_P (type))
+		{
+		  /* For floating point values, punt if this folding
+		     doesn't preserve bit representation (canonicalizes some
+		     bits e.g. in NaN, etc.), see PR95450.  */
+		  unsigned char ver[MAX_BITSIZE_MODE_ANY_MODE / BITS_PER_UNIT];
+		  if (native_encode_initializer (ret, ver, len, 0) != len
+		      || memcmp (buf, ver, len) != 0)
+		    ret = NULL_TREE;
+		}
+	    }
 	}
 
       return ret;
--- gcc/testsuite/gcc.target/powerpc/pr95450.c.jj	2020-08-11 19:21:35.654633211 +0200
+++ gcc/testsuite/gcc.target/powerpc/pr95450.c	2020-08-11 19:23:24.176147695 +0200
@@ -0,0 +1,29 @@
+/* PR target/95450 */
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-optimized" } */
+/* { dg-final { scan-tree-dump-not "return \[0-9.e+]\+;" "optimized" } } */
+
+/* Verify this is not optimized for double double into return floating_point_constant,
+   as while that constant is the maximum normalized floating point value, it needs
+   107 bit precision, which is more than GCC supports for this format.  */
+
+#if __LDBL_MANT_DIG__ == 106
+union U
+{
+  struct { double hi; double lo; } dd;
+  long double ld;
+};
+
+const union U g = { { __DBL_MAX__, __DBL_MAX__ / (double)134217728UL / (double)134217728UL } };
+#else
+struct S
+{
+  long double ld;
+} g;
+#endif
+
+long double
+foo (void)
+{
+  return g.ld;
+}

	Jakub