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". > > > > > 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.) -Eli
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