[llvm-dev] [RFC] Using basic block attributes to implement non-default floating point environment
Finkel, Hal J. via llvm-dev
llvm-dev at lists.llvm.org
Wed Oct 2 15:12:36 PDT 2019
On 10/1/19 12:35 AM, Serge Pavlov via llvm-dev wrote:
This proposal is aimed at support of floating point environment, in which some properties like rounding mode or exception behavior differ from those used by default. This include in particular support of 'pragma STDC FENV_ACCESS', 'pragma STDC FENV_ROUND' as well as some other related facilities.
On many processors use of non-default floating mode requires modification of global state by writing into some register. It presents a difficulty for implementation as a floating point instruction must not be move to code which executes with different floating point state. To prevent from such moves, the current solution represents FP operations with special (constrained) instructions, which do not participate in optimizations (http://lists.llvm.org/pipermail/cfe-dev/2017-August/055325.html). It is important that the constrained FP operations must be used everywhere in entire function including inlined calls, if they are used in some part of it.
The main concern about such approach is performance drop. Using constrained FP operations means that optimizations on FP operations are turned off, this is the main reason of using them. Even if non-default FP environment is used in a small piece of a function, optimizations are turned off in entire function. For many practical application this is unacceptable.
The reason, as you're likely aware, that the constrained FP operations must be used within the entire function is that, if you mix the constrained FP operations with the normal ones, there's no way to prevent code motion from intermixing them. The solution I recall being discussed to this problem of a function which requires constrained operations only in part is outlining in Clang - this does introduce function-call overhead (although perhaps some MI-level inlining pass could mitigate that in part), but otherwise permits normal optimization of the normal FP operations.
Although this approach prevents from moving instructions, it does not prevent from moving basic blocks. The code that uses non-default FP environment at some point must set appropriate state registers, do necessary operations and then restore the original mode. If this activity is scattered by several basic blocks, block-level optimizations can break these arrangement, for instance a basic block with default FP operations can be moved after the block that sets non-default FP environment.
Can you please provide some pseudocode to illustrate this problem? Moving basic blocks moves the instructions within them, and I don't see how our current semantics would prevent illegal reorderings of the instructions but not prevent illegal reorderings of groups of those same instructions. At the LLVM level, we currently model the FP-environment state as a kind of memory, and so the operations which adjust the FP-environment state must also be marked as writing to memory, but that's true with essentially all external program state, and that should prevent all illegal reordering.
The proposed approach is based on extension of basic blocks. It is assumed that code in basic block is executed in the same FP environment. The assumption is consistent with the rules of using 'pragma STDC FENV_ACCESS' and similar facilities. If the environment differs from default, such block has pointer to some object that keeps the block attributes including FP settings. All basic blocks, obtained from the same block where 'pragma STDC FENV_ACCESS' is specified, share the same attribute object. In bytecode these attributes are represented by metadata attached to the basic blocks.
With basic block attributes compiler can assert validity of an instruction move by comparing attributes of source and destination BBs. An instruction should keep pointer to BB attributes even if it is detached from BB, to support common technique of moving instructions. Similarly compiler can verify validity of BB movement.
Such approach allows to develop implementation in which constrained FP operations are 'jailed' in their basic blocks. Other part of the function can still use usual FP operations and get profit of optimizations. Depending on the target hardware some FP operations may be allowed to cross the 'jail' boundary, for instance, it they correspond to instructions which directly encode rounding mode and FP environment change rounding mode only.
Is this solution feasible? What are obstacles, difficulties or drawbacks for it? Are there any improvements for it? Any feedback is welcome.
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