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Jakub Jelinek [Tue, 25 Aug 2020 05:17:10 +0000 (07:17 +0200)]

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

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.

I think the verification that what we encode can be interpreted back
woiuld be only an internal consistency check (so perhaps for ENABLE_CHECKING
if flag_checking only, but if both directions perform it, then we need
to avoid mutual recursion).
While for the other direction (interpretation), at least for the broken by
design long doubles we just know we can't represent in GCC all valid values.
The other floating point formats are just theoretical case, perhaps we would
canonicalize something to a value that wouldn't trigger invalid exception
when without canonicalization it would trigger it at runtime, so let's just
ignore those.

Adjusted (so far untested) patch to do it in native_interpret_real instead
and limit it to the MODE_COMPOSITE_P cases, for which e.g.
fold-const.c/simplify-rtx.c punts in several other places too because we just
know we can't represent everything.

E.g.
      /* Don't constant fold this floating point operation if the
         result may dependent upon the run-time rounding mode and
         flag_rounding_math is set, or if GCC's software emulation
         is unable to accurately represent the result.  */
      if ((flag_rounding_math
           || (MODE_COMPOSITE_P (mode) && !flag_unsafe_math_optimizations))
          && (inexact || !real_identical (&result, &value)))
        return NULL_TREE;
Or perhaps guard it with MODE_COMPOSITE_P (mode) && !flag_unsafe_math_optimizations
too, thus break what gnulib / m4 does with -ffast-math, but not normally?

2020-08-25  Jakub Jelinek  <jakub@redhat.com>

PR target/95450
* fold-const.c (native_interpret_real): For MODE_COMPOSITE_P modes
punt if the to be returned REAL_CST does not encode to the bitwise
same representation.

* gcc.target/powerpc/pr95450.c: New test.