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When it was first created, CFGSort only made sure BBs in each
`MachineLoop` are sorted together. After we added exception support,
CFGSort now also sorts BBs in each `WebAssemblyException`, which
represents a `catch` block, together, and
`Region` class was introduced to be a thin wrapper for both
`MachineLoop` and `WebAssemblyException`.
But how we compute those loops and exceptions is different.
`MachineLoopInfo` is constructed using the standard loop computation
algorithm in LLVM; the definition of loop is "a set of BBs that are
dominated by a loop header and have a path back to the loop header". So
even if some BBs are semantically contained by a loop in the original
program, or in other words dominated by a loop header, if they don't
have a path back to the loop header, they are not considered a part of
the loop. For example, if a BB is dominated by a loop header but
contains `call abort()` or `rethrow`, it wouldn't have a path back to
the header, so it is not included in the loop.
But `WebAssemblyException` is wasm-specific data structure, and its
algorithm is simple: a `WebAssemblyException` consists of an EH pad and
all BBs dominated by the EH pad. So this scenario is possible: (This is
also the situation in the newly added test in cfg-stackify-eh.ll)
```
Loop L: header, A, ehpad, latch
Exception E: ehpad, latch, B
```
(B contains `abort()`, so it does not have a path back to the loop
header, so it is not included in L.)
And it is sorted in this order:
```
header
A
ehpad
latch
B
```
And when CFGStackify places `end_loop` or `end_try` markers, it
previously used `WebAssembly::getBottom()`, which returns the latest BB
in the sorted order, and placed the marker there. So in this case the
marker placements will be like this:
```
loop
header
try
A
catch
ehpad
latch
end_loop <-- misplaced!
B
end_try
```
in which nesting between the loop and the exception is not correct.
`end_loop` marker has to be placed after `B`, and also after `end_try`.
Maybe the fundamental way to solve this problem is to come up with our
own algorithm for computing loop region too, in which we include all BBs
dominated by a loop header in a loop. But this takes a lot more effort.
The only thing we need to fix is actually, `getBottom()`. If we make it
return the right BB, which means in case of a loop, the latest BB of the
loop itself and all exceptions contained in there, we are good.
This renames `Region` and `RegionInfo` to `SortRegion` and
`SortRegionInfo` and extracts them into their own file. And add
`getBottom` to `SortRegionInfo` class, from which it can access
`WebAssemblyExceptionInfo`, so that it can compute a correct bottom
block for loops.
Reviewed By: dschuff
Differential Revision: https://reviews.llvm.org/D84724
92 lines
3.0 KiB
C++
92 lines
3.0 KiB
C++
//===-- WebAssemblySortRegion.h - WebAssembly Sort SortRegion ----*- C++-*-===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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///
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/// \file
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/// \brief This file implements regions used in CFGSort and CFGStackify.
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///
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//===----------------------------------------------------------------------===//
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#ifndef LLVM_LIB_TARGET_WEBASSEMBLY_WEBASSEMBLYSORTREGION_H
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#define LLVM_LIB_TARGET_WEBASSEMBLY_WEBASSEMBLYSORTREGION_H
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#include "llvm/ADT/ArrayRef.h"
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#include "llvm/ADT/DenseMap.h"
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#include "llvm/ADT/iterator_range.h"
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namespace llvm {
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class MachineBasicBlock;
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class MachineLoop;
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class MachineLoopInfo;
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class WebAssemblyException;
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class WebAssemblyExceptionInfo;
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namespace WebAssembly {
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// Wrapper for loops and exceptions
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class SortRegion {
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public:
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virtual ~SortRegion() = default;
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virtual MachineBasicBlock *getHeader() const = 0;
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virtual bool contains(const MachineBasicBlock *MBB) const = 0;
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virtual unsigned getNumBlocks() const = 0;
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using block_iterator = typename ArrayRef<MachineBasicBlock *>::const_iterator;
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virtual iterator_range<block_iterator> blocks() const = 0;
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virtual bool isLoop() const = 0;
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};
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template <typename T> class ConcreteSortRegion : public SortRegion {
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const T *Unit;
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public:
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ConcreteSortRegion(const T *Unit) : Unit(Unit) {}
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MachineBasicBlock *getHeader() const override { return Unit->getHeader(); }
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bool contains(const MachineBasicBlock *MBB) const override {
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return Unit->contains(MBB);
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}
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unsigned getNumBlocks() const override { return Unit->getNumBlocks(); }
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iterator_range<block_iterator> blocks() const override {
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return Unit->blocks();
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}
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bool isLoop() const override { return false; }
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};
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// This class has information of nested SortRegions; this is analogous to what
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// LoopInfo is for loops.
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class SortRegionInfo {
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friend class ConcreteSortRegion<MachineLoopInfo>;
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friend class ConcreteSortRegion<WebAssemblyException>;
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const MachineLoopInfo &MLI;
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const WebAssemblyExceptionInfo &WEI;
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DenseMap<const MachineLoop *, std::unique_ptr<SortRegion>> LoopMap;
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DenseMap<const WebAssemblyException *, std::unique_ptr<SortRegion>>
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ExceptionMap;
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public:
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SortRegionInfo(const MachineLoopInfo &MLI,
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const WebAssemblyExceptionInfo &WEI)
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: MLI(MLI), WEI(WEI) {}
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// Returns a smallest loop or exception that contains MBB
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const SortRegion *getRegionFor(const MachineBasicBlock *MBB);
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// Return the "bottom" block among all blocks dominated by the region
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// (MachineLoop or WebAssemblyException) header. This works when the entity is
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// discontiguous.
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MachineBasicBlock *getBottom(const SortRegion *R);
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MachineBasicBlock *getBottom(const MachineLoop *ML);
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MachineBasicBlock *getBottom(const WebAssemblyException *WE);
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};
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} // end namespace WebAssembly
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} // end namespace llvm
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#endif
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