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[WebAssembly] Fix catch unwind mismatches 

This fixes unwind destination mismatches caused by 'catch'es, which
occur when a foreign exception is not caught by the nearest `catch` and
the next outer `catch` is not the catch it should unwind to, or the next
unwind destination should be the caller instead. This kind of mismatches
didn't exist in the previous version of the spec, because in the
previous spec `catch` was effectively `catch_all`, catching all
exceptions.

Reviewed By: tlively

Differential Revision: https://reviews.llvm.org/D94049
This commit is contained in:
Heejin Ahn 2020-12-30 22:29:38 -08:00
parent ed41945faa
commit 9f770b36cb
2 changed files with 226 additions and 53 deletions
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196
llvm/lib/Target/WebAssembly/WebAssemblyCFGStackify.cpp
View File

@ -31,6 +31,7 @@
#include "llvm/CodeGen/MachineDominators.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineLoopInfo.h"
#include "llvm/CodeGen/WasmEHFuncInfo.h"
#include "llvm/MC/MCAsmInfo.h"
#include "llvm/Target/TargetMachine.h"
using namespace llvm;
@ -39,6 +40,7 @@ using WebAssembly::SortRegionInfo;
#define DEBUG_TYPE "wasm-cfg-stackify"
STATISTIC(NumCallUnwindMismatches, "Number of call unwind mismatches found");
STATISTIC(NumCatchUnwindMismatches, "Number of catch unwind mismatches found");
namespace {
class WebAssemblyCFGStackify final : public MachineFunctionPass {
@ -898,26 +900,70 @@ void WebAssemblyCFGStackify::addTryDelegate(MachineInstr *RangeBegin,
EndBB->addSuccessor(DelegateBB);
} else {
// If the range's end instruction is in the middle of the BB, we split the
// BB into two and insert the delegate BB in between.
// - Before:
// bb:
// range_end
// other_insts
//
// - After:
// pre_bb: (previous 'bb')
// range_end
// delegate_bb: (new)
// delegate
// post_bb: (new)
// other_insts
MachineBasicBlock *PreBB = EndBB;
MachineBasicBlock *PostBB = MF.CreateMachineBasicBlock();
MF.insert(std::next(PreBB->getIterator()), PostBB);
MF.insert(std::next(PreBB->getIterator()), DelegateBB);
PostBB->splice(PostBB->end(), PreBB, SplitPos, PreBB->end());
PostBB->transferSuccessors(PreBB);
// When the split pos is in the middle of a BB, we split the BB into two and
// put the 'delegate' BB in between. We normally create a split BB and make
// it a successor of the original BB (PostSplit == true), but in case the BB
// is an EH pad and the split pos is before 'catch', we should preserve the
// BB's property, including that it is an EH pad, in the later part of the
// BB, where 'catch' is. In this case we set PostSplit to false.
bool PostSplit = true;
if (EndBB->isEHPad()) {
for (auto I = MachineBasicBlock::iterator(SplitPos), E = EndBB->end();
I != E; ++I) {
if (WebAssembly::isCatch(I->getOpcode())) {
PostSplit = false;
break;
}
}
}
MachineBasicBlock *PreBB = nullptr, *PostBB = nullptr;
if (PostSplit) {
// If the range's end instruction is in the middle of the BB, we split the
// BB into two and insert the delegate BB in between.
// - Before:
// bb:
// range_end
// other_insts
//
// - After:
// pre_bb: (previous 'bb')
// range_end
// delegate_bb: (new)
// delegate
// post_bb: (new)
// other_insts
PreBB = EndBB;
PostBB = MF.CreateMachineBasicBlock();
MF.insert(std::next(PreBB->getIterator()), PostBB);
MF.insert(std::next(PreBB->getIterator()), DelegateBB);
PostBB->splice(PostBB->end(), PreBB, SplitPos, PreBB->end());
PostBB->transferSuccessors(PreBB);
} else {
// - Before:
// ehpad:
// range_end
// catch
// ...
//
// - After:
// pre_bb: (new)
// range_end
// delegate_bb: (new)
// delegate
// post_bb: (previous 'ehpad')
// catch
// ...
assert(EndBB->isEHPad());
PreBB = MF.CreateMachineBasicBlock();
PostBB = EndBB;
MF.insert(PostBB->getIterator(), PreBB);
MF.insert(PostBB->getIterator(), DelegateBB);
PreBB->splice(PreBB->end(), PostBB, PostBB->begin(), SplitPos);
// We don't need to transfer predecessors of the EH pad to 'PreBB',
// because an EH pad's predecessors are all through unwind edges and they
// should still unwind to the EH pad, not PreBB.
}
unstackifyVRegsUsedInSplitBB(*PreBB, *PostBB);
PreBB->addSuccessor(DelegateBB);
PreBB->addSuccessor(PostBB);
@ -992,7 +1038,6 @@ bool WebAssemblyCFGStackify::fixCallUnwindMismatches(MachineFunction &MF) {
// call with an inner try-delegate that rethrows the exception to the right
// 'catch'.
//
//
// try
// try
// call @foo
@ -1219,8 +1264,113 @@ bool WebAssemblyCFGStackify::fixCallUnwindMismatches(MachineFunction &MF) {
}
bool WebAssemblyCFGStackify::fixCatchUnwindMismatches(MachineFunction &MF) {
// TODO implement
return false;
// There is another kind of unwind destination mismatches besides call unwind
// mismatches, which we will call "catch unwind mismatches". See this example
// after the marker placement:
// try
// try
// call @foo
// catch __cpp_exception ;; ehpad A (next unwind dest: caller)
// ...
// end_try
// catch_all ;; ehpad B
// ...
// end_try
//
// 'call @foo's unwind destination is the ehpad A. But suppose 'call @foo'
// throws a foreign exception that is not caught by ehpad A, and its next
// destination should be the caller. But after control flow linearization,
// another EH pad can be placed in between (e.g. ehpad B here), making the
// next unwind destination incorrect. In this case, the foreign exception
// will instead go to ehpad B and will be caught there instead. In this
// example the correct next unwind destination is the caller, but it can be
// another outer catch in other cases.
//
// There is no specific 'call' or 'throw' instruction to wrap with a
// try-delegate, so we wrap the whole try-catch-end with a try-delegate and
// make it rethrow to the right destination, as in the example below:
// try
// try ;; (new)
// try
// call @foo
// catch __cpp_exception ;; ehpad A (next unwind dest: caller)
// ...
// end_try
// delegate 1 (caller) ;; (new)
// catch_all ;; ehpad B
// ...
// end_try
const auto *EHInfo = MF.getWasmEHFuncInfo();
SmallVector<const MachineBasicBlock *, 8> EHPadStack;
// For EH pads that have catch unwind mismatches, a map of <EH pad, its
// correct unwind destination>.
DenseMap<MachineBasicBlock *, MachineBasicBlock *> EHPadToUnwindDest;
for (auto &MBB : reverse(MF)) {
for (auto &MI : reverse(MBB)) {
if (MI.getOpcode() == WebAssembly::TRY)
EHPadStack.pop_back();
else if (MI.getOpcode() == WebAssembly::DELEGATE)
EHPadStack.push_back(&MBB);
else if (WebAssembly::isCatch(MI.getOpcode())) {
auto *EHPad = &MBB;
// catch_all always catches an exception, so we don't need to do
// anything
if (MI.getOpcode() == WebAssembly::CATCH_ALL) {
}
// This can happen when the unwind dest was removed during the
// optimization, e.g. because it was unreachable.
else if (EHPadStack.empty() && EHInfo->hasEHPadUnwindDest(EHPad)) {
LLVM_DEBUG(dbgs() << "EHPad (" << EHPad->getName()
<< "'s unwind destination does not exist anymore"
<< "\n\n");
}
// The EHPad's next unwind destination is the caller, but we incorrectly
// unwind to another EH pad.
else if (!EHPadStack.empty() && !EHInfo->hasEHPadUnwindDest(EHPad)) {
EHPadToUnwindDest[EHPad] = getFakeCallerBlock(MF);
LLVM_DEBUG(dbgs()
<< "- Catch unwind mismatch:\nEHPad = " << EHPad->getName()
<< " Original dest = caller Current dest = "
<< EHPadStack.back()->getName() << "\n\n");
}
// The EHPad's next unwind destination is an EH pad, whereas we
// incorrectly unwind to another EH pad.
else if (!EHPadStack.empty() && EHInfo->hasEHPadUnwindDest(EHPad)) {
auto *UnwindDest = EHInfo->getEHPadUnwindDest(EHPad);
if (EHPadStack.back() != UnwindDest) {
EHPadToUnwindDest[EHPad] = UnwindDest;
LLVM_DEBUG(dbgs() << "- Catch unwind mismatch:\nEHPad = "
<< EHPad->getName() << " Original dest = "
<< UnwindDest->getName() << " Current dest = "
<< EHPadStack.back()->getName() << "\n\n");
}
}
EHPadStack.push_back(EHPad);
}
}
}
assert(EHPadStack.empty());
if (EHPadToUnwindDest.empty())
return false;
NumCatchUnwindMismatches += EHPadToUnwindDest.size();
for (auto &P : EHPadToUnwindDest) {
MachineBasicBlock *EHPad = P.first;
MachineBasicBlock *UnwindDest = P.second;
MachineInstr *Try = EHPadToTry[EHPad];
MachineInstr *EndTry = BeginToEnd[Try];
addTryDelegate(Try, EndTry, UnwindDest);
}
return true;
}
void WebAssemblyCFGStackify::recalculateScopeTops(MachineFunction &MF) {

83
llvm/test/CodeGen/WebAssembly/cfg-stackify-eh.ll
View File

@ -379,22 +379,33 @@ try.cont: ; preds = %catch.start, %loop
; destination mismatches. And we use -wasm-disable-ehpad-sort to create maximum
; number of mismatches in several tests below.
; - Call unwind mismatch
; 'call bar''s original unwind destination was 'C0', but after control flow
; linearization, its unwind destination incorrectly becomes 'C1'. We fix this by
; wrapping the call with a nested try-delegate that targets 'C0'.
; - Catch unwind mismatch
; If 'call foo' throws a foreign exception, it will not be caught by C1, and
; should be rethrown to the caller. But after control flow linearization, it
; will instead unwind to C0, an incorrect next EH pad. We wrap the whole
; try-catch with try-delegate that rethrows an exception to the caller to fix
; this.
; NOSORT-LABEL: test5
; NOSORT: try
; NOSORT: try
; NOSORT: call foo
; --- try-delegate starts (call unwind mismatch)
; --- try-delegate starts (catch unwind mismatch)
; NOSORT try
; NOSORT: try
; NOSORT: call bar
; NOSORT: delegate 1 # label/catch{{[0-9]+}}: down to catch[[C0:[0-9]+]]
; NOSORT: call foo
; --- try-delegate starts (call unwind mismatch)
; NOSORT: try
; NOSORT: call bar
; NOSORT: delegate 2 # label/catch{{[0-9]+}}: down to catch[[C0:[0-9]+]]
; --- try-delegate ends (call unwind mismatch)
; NOSORT: catch {{.*}} # catch[[C1:[0-9]+]]:
; NOSORT: end_try
; NOSORT: catch {{.*}} # catch[[C0]]:
; NOSORT: catch {{.*}} # catch[[C1:[0-9]+]]:
; NOSORT: end_try
; NOSORT: delegate 1 # label/catch{{[0-9]+}}: to caller
; --- try-delegate ends (catch unwind mismatch)
; NOSORT: catch {{.*}} # catch[[C0]]:
; NOSORT: end_try
; NOSORT: return
@ -483,20 +494,24 @@ try.cont: ; preds = %catch.start0
; NOSORT-LABEL: test7
; NOSORT: try
; NOSORT: try
; NOSORT: call foo
; --- try-delegate starts (call unwind mismatch)
; --- try-delegate starts (catch unwind mismatch)
; NOSORT try
; NOSORT: try
; NOSORT: call foo
; NOSORT: delegate 2 # label/catch{{[0-9]+}}: to caller
; --- try-delegate starts (call unwind mismatch)
; NOSORT: try
; NOSORT: call foo
; NOSORT: delegate 3 # label/catch{{[0-9]+}}: to caller
; --- try-delegate ends (call unwind mismatch)
; --- try-delegate starts (call unwind mismatch)
; NOSORT: try
; NOSORT: call bar
; NOSORT: delegate 1 # label/catch{{[0-9]+}}: down to catch[[C0:[0-9]+]]
; NOSORT: try
; NOSORT: call bar
; NOSORT: delegate 2 # label/catch{{[0-9]+}}: down to catch[[C0:[0-9]+]]
; --- try-delegate ends (call unwind mismatch)
; NOSORT: catch {{.*}} # catch[[C1:[0-9]+]]:
; NOSORT: end_try
; NOSORT: catch {{.*}} # catch[[C1:[0-9]+]]:
; NOSORT: end_try
; NOSORT: delegate 1 # label/catch{{[0-9]+}}: to caller
; --- try-delegate ends (catch unwind mismatch)
; NOSORT: catch {{.*}} # catch[[C0]]:
; NOSORT: end_try
; NOSORT: return
@ -618,30 +633,37 @@ try.cont: ; preds = %catch.start0
ret void
}
; When we have both kinds of EH pad unwind mismatches:
; We have two call unwind unwind mismatches:
; - A may-throw instruction unwinds to an incorrect EH pad after linearizing the
; CFG, when it is supposed to unwind to another EH pad.
; - A may-throw instruction unwinds to an incorrect EH pad after linearizing the
; CFG, when it is supposed to unwind to the caller.
; We also have a catch unwind mismatch: If an exception is not caught by the
; first catch because it is a non-C++ exception, it shouldn't unwind to the next
; catch, but it should unwind to the caller.
; NOSORT-LABEL: test10
; NOSORT: try
; --- try-delegate starts (catch unwind mismatch)
; NOSORT: try
; NOSORT: call foo
; --- try-delegate starts (call unwind mismatch)
; NOSORT: try
; NOSORT: call bar
; NOSORT: delegate 1 # label/catch{{[0-9]+}}: down to catch[[C0:[0-9]+]]
; --- try-delegate ends (call unwind mismatch)
; NOSORT: catch
; NOSORT: call {{.*}} __cxa_begin_catch
; NOSORT: call foo
; --- try-delegate starts (call unwind mismatch)
; NOSORT: try
; NOSORT: call __cxa_end_catch
; NOSORT: delegate 1 # label/catch{{[0-9]+}}: to caller
; NOSORT: try
; NOSORT: call bar
; NOSORT: delegate 2 # label/catch{{[0-9]+}}: down to catch[[C0:[0-9]+]]
; --- try-delegate ends (call unwind mismatch)
; NOSORT: end_try
; NOSORT: catch {{.*}} # catch[[C0]]:
; NOSORT: catch
; NOSORT: call {{.*}} __cxa_begin_catch
; --- try-delegate starts (call unwind mismatch)
; NOSORT: try
; NOSORT: call __cxa_end_catch
; NOSORT: delegate 2 # label/catch{{[0-9]+}}: to caller
; --- try-delegate ends (call unwind mismatch)
; NOSORT: end_try
; NOSORT: delegate 1 # label/catch{{[0-9]+}}: to caller
; --- try-delegate ends (catch unwind mismatch)
; NOSORT: catch {{.*}} # catch[[C0]]:
; NOSORT: call {{.*}} __cxa_begin_catch
; NOSORT: call __cxa_end_catch
; NOSORT: end_try
@ -1079,6 +1101,7 @@ ehcleanup: ; preds = %if.then
; Check if the unwind destination mismatch stats are correct
; NOSORT: 18 wasm-cfg-stackify - Number of call unwind mismatches found
; NOSORT: 3 wasm-cfg-stackify - Number of catch unwind mismatches found
declare void @foo()
declare void @bar()