----- Original Message ----- > > On Thu, Sep 12, 2013 at 2:44 PM, Hal Finkel < [email protected] > > wrote: > > > > > > ----- Original Message ----- > > > > On Thu, Sep 12, 2013 at 2:03 PM, Hal Finkel < [email protected] > > > wrote: > > > > > > > > > > Hello, > > > > Please review the attached patch which restores the libm sqrt* -> > > @llvm.sqrt* mapping, but only in fast-math mode (specifically, when > > the UnsafeFPMath or NoNaNsFPMath CodeGen options are enabled). The > > @llvm.sqrt* intrinsics have slightly different semantics from the > > libm call, specifically, they are undefined when given a non-zero > > negative number (the libm calls will always return NaN for any > > negative number). > > > > This mapping was removed in r100613, and replaced with a TODO, but > > at > > that time the fast-math flags were not yet implemented. Now that we > > have these, restoring this mapping is important because it will > > enable autovectorization of sqrt calls in loops (at least in > > fast-math mode). > > > > > > > > > > This is dangerous, if LangRef is actually correct. People don't > > associate -ffast-math with "my program will crash at random". :) Of > > course, LangRef is probably overstating the issue. > > I agree, and the LangRef does indeed say "undefined behavior", but I > assume that should really mean, "returns an undefined value." Do you > agree? > > > > Well, if we map llvm.sqrt to sqrt and sqrt sets errno, we really do > mean "undefined behavior"... or at least something more that > "returns an undefined value".
Agreed, but this possibility-of-setting-errno problem exists for all LLVM libm-style intrinsics, and so also exists for pow() [for which we currently do this exact kind of replacement whenever -fmath-errno=0]. So, FWIW, this is not without precedent. > > > > > > > > > > That said, there's actually a general issue here: if we map the > > LLVM > > intrinsics to libc functions, and the libc functions set errno, we > > could break code that depends on errno for non-math calls (e.g. > > fopen().) > > Perhaps, but I'm not changing that here. For one thing, if the > mapping does, in effect, sqrt -> llvm.sqrt -> sqrt, and only when > -fmath-errno=0. Are you worried about cases where the libm functions > actually do set errno (even though we have -fmath-errno=0)? > > > > > I'm not sure our implementation of -fno-math-errno is safe: according > to the gcc manual, it isn't equivalent to marking the math functions > with attribute((const)). (For example, the gcc manual's definition > allows transforming a call to sqrt() into the SSE sqrt instruction, > but it doesn't allow hoisting a call to sqrt out of arbitrary loops > on a machine where the sqrt() call could set errno.) I'm sure it is not safe, for the very reason that you highlight. Nevertheless, this is a long-standing problem, affecting the implementation of -fmath-errno=0 on all systems for which libm math functions actually do set errno, and will require a general solution (fairly orthogonal to this patch). I recommend that we: 1. Commit this change (so that we can autovectorize calls to sqrt()). 2. Have a discussion about how to actually solve this problem: I think that it involves making a specific function attribute for setting errno, and teaching the alias analysis infrastructure to do something sensible with it. -Hal > > > -Eli -- Hal Finkel Assistant Computational Scientist Leadership Computing Facility Argonne National Laboratory _______________________________________________ cfe-commits mailing list [email protected] http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits
