2019 EuroLLVM Developers’ Meeting: M. Berg & S. Canon “LLVM Numerics Improvements”

http://llvm.org/devmtg/2019-04/
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LLVM Numerics Improvements - Michael Berg (Apple), Steve Canon (Apple)

Slides: http://llvm.org/devmtg/2019-04/slides...
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Some LLVM based compilers currently provide two modes of floating point code generation. The first mode, called fast-math, is where performance is the primary consideration over numerical precision and accuracy. This mode does not strictly follow the IEEE-754 standard, but has proven useful for applications that do not require this level of precision. The second mode, called precise-math, is where the compiler carefully follows the subset of behavior defined in the IEEE standard that is applicable to conforming hardware targets. This mode is primarily used for compute workloads and wherever fast-math precision is inadequate, however it runs much slower as it requires a larger number of instructions in general. In practice neither of these modes is particularly desirable. The fast-math mode ignores a significant portion of the standard as pertains to handling undefined values described as Not a Number (NaNs) and Infinities (INFs), resulting in difficulties for certain workloads when the hardware target computes these values correctly and performance remains critical.

Until recently these two models were mutually exclusive, however with the addition of IR flags they need not be. For instance, the FastMath metadata module flag drives behavior deemed numerically unsafe when it is enabled, by indiscriminately enabling optimizations. With IR flags this behavior can be enabled with much finer granularity, allowing various code forms to be fast or precise together in one module. We call this mixed mode compilation. IR flags can be used individually or paired to produce desired floating point behavior under specified constraints with fine granularity of control. Optimization passes have been modified under this new kind of control to produce this behavior. This talk will describe the recent numerics work and discuss the implications for front-ends and backends built with LLVM.
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