Date: Sun, 9 Feb 2020 12:02:01 -0500 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Excess precision hell Recent finds in GCC bug tracker and experimentation with GCC and clang have me rather concerned about the safety of code built for i386. Some background references: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85957 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=323 At one point I'd almost thought we should remove -ffloat-store as a fallback for -fexcess-precision=standard on old GCCs, but it turns out that's a really bad idea. GCC (perhaps wrongly? this isn't clear from psABI) assumes that function call results don't have excess precision, so generating a libc.a/libc.so where they can return excess precision is dangerous (lies to the optimizer, leading to things like 85957 above). Using -ffloat-store produces results that are numerically wrong (not to mention slow), but at least consistent and deterministic. Unfortunately clang does not support either -ffloat-store or -fexcess-precision=standard, so **all** versions of musl build for i386 by clang are seriously broken in this regard. The only way I've found to make clang drop excess precision is by casting/coercing (by return statement) down from long double to double or float. (And note that a simple gratuitous cast up/down doesn't help; there actually needs to be a floating point operation that's been evaluated in the higher precision.) So, our code that's using float_t/double_t internally is presumably clang-safe, but there's a lot that's not doing that yet -- single precision hyperbolic functions and special functions, all complex math, maybe other things too. We probably should deprecate or disallow building of i386 musl with clang (unless -march is such that sse2 math is available and can be used, in which case there's no excess precision and everything is fine) unless/until everything is converted to use of float_t/double_t, or conditional on a test for a fix in clang. (For example we could write a configure test that disallows use of clang if x+y generates fadd but not fst[p]l in the asm.) Does this sound reasonable? One thing I just found, but I don't know if it's reliable: it seems clang's -O0 gives the equivalent of -ffloat-store. So perhaps we could just force -O0 (just for src/math and src/complex?) if clang is detected as having this bug. Rich
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