Avoid copying out/inout args when inlining functions #4877
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While it would be nice to cover more cases, this is a useful start as it is. The main thing it does not catch is when a control calls a (global) function with a local or param of the control as an inout argument. That would be nice to catch, but would require determining that the control does not do something with that local/param that will result in it effectively being aliased to something else in the global function. |
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- If the actual argument for an out or inout argument is local to the calling functions, we can use it directly in the inlined function instead of introducing extra copies in and out. Signed-off-by: Chris Dodd <cdodd@nvidia.com>
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This is a proposal/work in progress for a way of reducing the number of copies introduced when inlining.
On particularly problematic case is when you have a big struct (eg, all the headers) that is passed around to functions as an
inoutargument so they can access/modify it. Introducing a copy of all the headers is particularly inefficient, and tough to later optimize away if it was unnecessary.This change just has a minimal check -- if the actual argument passed to an
inoutoroutargument is local to the caller, it can't possibly be accessed by the callee directly, so no copy is needed -- the inlined code can just access it directly.A more general check would be to actually look at the callee -- if it does not access whatever is passed as an argument directly or indirectly, then no copy is needed. The indirectly part is complex -- requires looking recursively at whatever the callee calls, including any extern functions or methods on instances.
Or perhaps there should be a target-specific policy that controls this -- would allow better target-specific understanding of externs and what they do.
The problem is connected to doing too much in the frontend -- function inlining happens in the frontend (when arguably it should not), in order to allow inlining into type params (
bit<N>andint<N>in particular). If we remove the general inlining in the frontend (and only do it in the midend), it would greatly help with target-specific policies and tweaks.