2 Commits

Author	SHA1	Message	Date
Fedor Sergeev	6660fd0f95	[PM][FunctionAttrs] add NoUnwind attribute inference to PostOrderFunctionAttrs pass ... Summary: This was motivated by absence of PrunEH functionality in new PM. It was decided that a proper way to do PruneEH is to add NoUnwind inference into PostOrderFunctionAttrs and then perform normal SimplifyCFG on top. This change generalizes attribute handling implemented for (a removal of) Convergent attribute, by introducing a generic builder-like class AttributeInferer It registers all the attribute inference requests, storing per-attribute predicates into a vector, and then goes through an SCC Node, scanning all the instructions for not breaking attribute assumptions. The main idea is that as soon all the instructions from all the functions of SCC Node conform to attribute assumptions then we are free to infer the attribute as set for all the functions of SCC Node. It handles two distinct cases of attributes: - those that might break due to derefinement of the function code for these attributes we are allowed to apply inference only if all the functions are "exact definitions". Example - NoUnwind. - those that do not care about derefinement for these attributes we are allowed to apply inference as soon as we see any function definition. Example - removal of Convergent attribute. Also in this commit: * Converted all the FunctionAttrs tests to use FileCheck and added new-PM invocations to them * FunctionAttrs/convergent.ll test demonstrates a difference in behavior between new and old PM implementations. Marked with FIXME. * PruneEH tests were converted to new-PM as well, using function-attrs+simplify-cfg combo as intended * some of "other" tests were updated since function-attrs now infers 'nounwind' even for old PM pipeline * -disable-nounwind-inference hidden option added as a possible workaround for a supposedly rare case when nounwind being inferred by default presents a problem Reviewers: chandlerc, jlebar Reviewed By: jlebar Subscribers: eraman, llvm-commits Differential Revision: https://reviews.llvm.org/D44415 llvm-svn: 328377	2018-03-23 21:46:16 +00:00
Sanjoy Das	5ce3272833	Don't IPO over functions that can be de-refined ... Summary: Fixes PR26774. If you're aware of the issue, feel free to skip the "Motivation" section and jump directly to "This patch". Motivation: I define "refinement" as discarding behaviors from a program that the optimizer has license to discard. So transforming: ``` void f(unsigned x) { unsigned t = 5 / x; (void)t; } ``` to ``` void f(unsigned x) { } ``` is refinement, since the behavior went from "if x == 0 then undefined else nothing" to "nothing" (the optimizer has license to discard undefined behavior). Refinement is a fundamental aspect of many mid-level optimizations done by LLVM. For instance, transforming `x == (x + 1)` to `false` also involves refinement since the expression's value went from "if x is `undef` then { `true` or `false` } else { `false` }" to "`false`" (by definition, the optimizer has license to fold `undef` to any non-`undef` value). Unfortunately, refinement implies that the optimizer cannot assume that the implementation of a function it can see has all of the behavior an unoptimized or a differently optimized version of the same function can have. This is a problem for functions with comdat linkage, where a function can be replaced by an unoptimized or a differently optimized version of the same source level function. For instance, FunctionAttrs cannot assume a comdat function is actually `readnone` even if it does not have any loads or stores in it; since there may have been loads and stores in the "original function" that were refined out in the currently visible variant, and at the link step the linker may in fact choose an implementation with a load or a store. As an example, consider a function that does two atomic loads from the same memory location, and writes to memory only if the two values are not equal. The optimizer is allowed to refine this function by first CSE'ing the two loads, and the folding the comparision to always report that the two values are equal. Such a refined variant will look like it is `readonly`. However, the unoptimized version of the function can still write to memory (since the two loads //can// result in different values), and selecting the unoptimized version at link time will retroactively invalidate transforms we may have done under the assumption that the function does not write to memory. Note: this is not just a problem with atomics or with linking differently optimized object files. See PR26774 for more realistic examples that involved neither. This patch: This change introduces a new set of linkage types, predicated as `GlobalValue::mayBeDerefined` that returns true if the linkage type allows a function to be replaced by a differently optimized variant at link time. It then changes a set of IPO passes to bail out if they see such a function. Reviewers: chandlerc, hfinkel, dexonsmith, joker.eph, rnk Subscribers: mcrosier, llvm-commits Differential Revision: http://reviews.llvm.org/D18634 llvm-svn: 265762	2016-04-08 00:48:30 +00:00