From 9f2f79df19fbfaa1c4be313c2f2b5ce04646433e Mon Sep 17 00:00:00 2001 From: Jakub Jelinek Date: Tue, 25 Aug 2020 07:17:10 +0200 Subject: [PATCH] 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 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. --- gcc/fold-const.c | 14 ++++++++++- gcc/testsuite/gcc.target/powerpc/pr95450.c | 29 ++++++++++++++++++++++ 2 files changed, 42 insertions(+), 1 deletion(-) create mode 100644 gcc/testsuite/gcc.target/powerpc/pr95450.c diff --git a/gcc/fold-const.c b/gcc/fold-const.c index 7c4d1eff215..78f72f0cfa3 100644 --- a/gcc/fold-const.c +++ b/gcc/fold-const.c @@ -8328,7 +8328,19 @@ native_interpret_real (tree type, const unsigned char *ptr, int len) } real_from_target (&r, tmp, mode); - return build_real (type, r); + tree ret = build_real (type, r); + if (MODE_COMPOSITE_P (mode)) + { + /* For floating point values in composite modes, punt if this folding + doesn't preserve bit representation. As the mode doesn't have fixed + precision while GCC pretends it does, there could be valid values that + GCC can't really represent accurately. See PR95450. */ + unsigned char buf[24]; + if (native_encode_expr (ret, buf, total_bytes, 0) != total_bytes + || memcmp (ptr, buf, total_bytes) != 0) + ret = NULL_TREE; + } + return ret; } diff --git a/gcc/testsuite/gcc.target/powerpc/pr95450.c b/gcc/testsuite/gcc.target/powerpc/pr95450.c new file mode 100644 index 00000000000..569d2b2c536 --- /dev/null +++ b/gcc/testsuite/gcc.target/powerpc/pr95450.c @@ -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; +} -- 2.30.2