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[WebAssembly] Fix try placement in fixing unwind mismatches

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Summary:
In CFGStackify, `fixUnwindMismatches` function fixes unwind destination
mismatches created by `try` marker placement. For example,
```
try
  ...
  call @qux  ;; This should throw to the caller!
catch
  ...
end
```
When `call @qux` is supposed to throw to the caller, it is possible that
it is wrapped inside a `catch` so in case it throws it ends up unwinding
there incorrectly. (Also it is possible `call @qux` is supposed to
unwind to another `catch` within the same function.)

To fix this, we wrap this inner `call @qux` with a nested
`try`-`catch`-`end` sequence, and within the nested `catch` body, branch
to the right destination:
```
block $l0
  try
    ...
    try                 ;; new nested try
      call @qux
    catch               ;; new nested catch
      local.set n       ;; store exnref to a local
      br $l0
    end
  catch
    ...
  end
end
local.get n             ;; retrieve exnref back
rethrow                 ;; rethrow to the caller
```

The previous algorithm placed the nested `try` right before the `call`.
But it is possible that there are stackified instructions before the
call from which the call takes arguments.
```
try
  ...
  i32.const 5
  call @qux  ;; This should throw to the caller!
catch
  ...
end
```

In this case we have to place `try` before those stackified
instructions.
```
block $l0
  try
    ...
    try                 ;; this should go *before* 'i32.const 5'
      i32.const 5
      call @qux
    catch
      local.set n
      br $l0
    end
  catch
    ...
  end
end
local.get n
rethrow
```

We correctly handle this in the first normal `try` placement phase
(`placeTryMarker` function), but failed to handle this in this
`fixUnwindMismatches`.

Reviewers: dschuff

Subscribers: sbc100, jgravelle-google, hiraditya, sunfish, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77950
This commit is contained in:
Heejin Ahn 2020-04-10 16:46:39 -07:00
parent 9b1a0d300e
commit ba40896f99
2 changed files with 100 additions and 29 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

23
llvm/lib/Target/WebAssembly/WebAssemblyCFGStackify.cpp
View File

@ -1109,6 +1109,9 @@ bool WebAssemblyCFGStackify::fixUnwindMismatches(MachineFunction &MF) {
MachineInstr *RangeBegin = nullptr, *RangeEnd = nullptr;
std::tie(RangeBegin, RangeEnd) = Range;
auto *MBB = RangeBegin->getParent();
// Store the first function call from this range, because RangeBegin can
// be moved to point EH_LABEL before the call
MachineInstr *RangeBeginCall = RangeBegin;
// Include possible EH_LABELs in the range
if (RangeBegin->getIterator() != MBB->begin() &&
@ -1126,9 +1129,27 @@ bool WebAssemblyCFGStackify::fixUnwindMismatches(MachineFunction &MF) {
}
}
// Local expression tree before the first call of this range should go
// after the nested TRY.
SmallPtrSet<const MachineInstr *, 4> AfterSet;
AfterSet.insert(RangeBegin);
AfterSet.insert(RangeBeginCall);
for (auto I = MachineBasicBlock::iterator(RangeBeginCall),
E = MBB->begin();
I != E; --I) {
if (std::prev(I)->isDebugInstr() || std::prev(I)->isPosition())
continue;
if (WebAssembly::isChild(*std::prev(I), MFI))
AfterSet.insert(&*std::prev(I));
else
break;
}
// Create the nested try instruction.
auto InsertPos = getLatestInsertPos(
MBB, SmallPtrSet<const MachineInstr *, 4>(), AfterSet);
MachineInstr *NestedTry =
BuildMI(*MBB, *RangeBegin, RangeBegin->getDebugLoc(),
BuildMI(*MBB, InsertPos, RangeBegin->getDebugLoc(),
TII.get(WebAssembly::TRY))
.addImm(int64_t(WebAssembly::BlockType::Void));

106
llvm/test/CodeGen/WebAssembly/cfg-stackify-eh.ll
View File

@ -483,16 +483,65 @@ try.cont: ; preds = %catch.start0
ret void
}
; Similar situation as @test6. Here 'call @qux''s original unwind destination
; was the caller, but after control flow linearization, their unwind destination
; incorrectly becomes another catch within the function. We fix this by wrapping
; the call with a nested try/catch/end_try and branching to the right
; destination, from which we rethrow the exception to the caller.
; Because 'call @qux' pops an argument pushed by 'i32.const 5' from stack, the
; nested 'try' should be placed before `i32.const 5', not between 'i32.const 5'
; and 'call @qux'.
; NOSORT-LABEL: test7
; NOSORT: try
; NOSORT: call foo
; --- Nested try/catch/end_try starts
; NOSORT: try
; NOSORT-NEXT: i32.const $push{{[0-9]+}}=, 5
; NOSORT-NEXT: call ${{[0-9]+}}=, qux
; NOSORT: catch $[[REG:[0-9]+]]=
; NOSORT: br 1 # 1: down to label37
; NOSORT: end_try
; --- Nested try/catch/end_try ends
; NOSORT: return
; NOSORT: catch $drop= # catch19:
; NOSORT: return
; NOSORT: end_try # label37:
; NOSORT: rethrow $[[REG]] # to caller
define i32 @test7() personality i8* bitcast (i32 (...)* @__gxx_wasm_personality_v0 to i8*) {
bb0:
invoke void @foo()
to label %bb1 unwind label %catch.dispatch0
bb1: ; preds = %bb0
%0 = call i32 @qux(i32 5)
ret i32 %0
catch.dispatch0: ; preds = %bb0
%1 = catchswitch within none [label %catch.start0] unwind to caller
catch.start0: ; preds = %catch.dispatch0
%2 = catchpad within %1 [i8* null]
%3 = call i8* @llvm.wasm.get.exception(token %2)
%j = call i32 @llvm.wasm.get.ehselector(token %2)
catchret from %2 to label %try.cont
try.cont: ; preds = %catch.start0
ret i32 0
}
; If not for the unwind destination mismatch, the LOOP marker here would have an
; i32 signature. But because we add a rethrow instruction at the end of the
; appendix block, now the LOOP marker does not have a signature (= has a void
; signature). Here the two calls two 'bar' are supposed to throw up to the
; caller, but incorrectly unwind to 'catch19' after linearizing the CFG.
; NOSORT-LABEL: test7
; NOSORT-LABEL: test8
; NOSORT: block
; NOSORT-NOT: loop i32
; NOSORT: loop # label38:
; NOSORT: loop # label40:
; NOSORT: try
; NOSORT: call foo
; --- Nested try/catch/end_try starts
@ -500,18 +549,18 @@ try.cont: ; preds = %catch.start0
; NOSORT: call bar
; NOSORT: call bar
; NOSORT: catch $[[REG:[0-9]+]]=
; NOSORT: br 1 # 1: down to label39
; NOSORT: br 1 # 1: down to label41
; NOSORT: end_try
; --- Nested try/catch/end_try ends
; NOSORT: return {{.*}}
; NOSORT: catch $drop= # catch19:
; NOSORT: br 1 # 1: up to label38
; NOSORT: end_try # label39:
; NOSORT: catch $drop= # catch21:
; NOSORT: br 1 # 1: up to label40
; NOSORT: end_try # label41:
; NOSORT: end_loop
; NOSORT: end_block
; NOSORT: rethrow $[[REG]] # to caller
define i32 @test7(i32* %p) personality i8* bitcast (i32 (...)* @__gxx_wasm_personality_v0 to i8*) {
define i32 @test8(i32* %p) personality i8* bitcast (i32 (...)* @__gxx_wasm_personality_v0 to i8*) {
entry:
store volatile i32 0, i32* %p
br label %loop
@ -545,7 +594,7 @@ try.cont: ; preds = %catch.start
; - A may-throw instruction unwinds to an incorrect EH pad after linearizing the
; CFG, when it is supposed to unwind to the caller.
; NOSORT-LABEL: test8
; NOSORT-LABEL: test9
; NOSORT: block
; NOSORT: block
; NOSORT: try
@ -555,40 +604,40 @@ try.cont: ; preds = %catch.start
; NOSORT: try
; NOSORT: call bar
; NOSORT: catch $[[REG0:[0-9]+]]=
; NOSORT: br 2 # 2: down to label43
; NOSORT: br 2 # 2: down to label45
; NOSORT: end_try
; --- Nested try/catch/end_try ends
; NOSORT: br 2 # 2: down to label42
; NOSORT: br 2 # 2: down to label44
; NOSORT: catch {{.*}}
; NOSORT: block i32
; NOSORT: br_on_exn 0, {{.*}} # 0: down to label46
; NOSORT: br_on_exn 0, {{.*}} # 0: down to label48
; --- Nested try/catch/end_try starts
; NOSORT: try
; NOSORT: rethrow {{.*}} # down to catch24
; NOSORT: catch $[[REG1:[0-9]+]]= # catch24:
; NOSORT: br 5 # 5: down to label41
; NOSORT: rethrow {{.*}} # down to catch26
; NOSORT: catch $[[REG1:[0-9]+]]= # catch26:
; NOSORT: br 5 # 5: down to label43
; NOSORT: end_try
; --- Nested try/catch/end_try ends
; NOSORT: end_block # label46:
; NOSORT: end_block # label48:
; NOSORT: call $drop=, __cxa_begin_catch
; --- Nested try/catch/end_try starts
; NOSORT: try
; NOSORT: call __cxa_end_catch
; NOSORT: catch $[[REG1]]=
; NOSORT: br 4 # 4: down to label41
; NOSORT: br 4 # 4: down to label43
; NOSORT: end_try
; --- Nested try/catch/end_try ends
; NOSORT: br 2 # 2: down to label42
; NOSORT: br 2 # 2: down to label44
; NOSORT: end_try
; NOSORT: catch $[[REG0]]=
; NOSORT: end_try # label43:
; NOSORT: end_try # label45:
; NOSORT: call $drop=, __cxa_begin_catch
; NOSORT: call __cxa_end_catch
; NOSORT: end_block # label42:
; NOSORT: end_block # label44:
; NOSORT: return
; NOSORT: end_block # label41:
; NOSORT: end_block # label43:
; NOSORT: rethrow $[[REG1]] # to caller
define void @test8() personality i8* bitcast (i32 (...)* @__gxx_wasm_personality_v0 to i8*) {
define void @test9() personality i8* bitcast (i32 (...)* @__gxx_wasm_personality_v0 to i8*) {
bb0:
invoke void @foo()
to label %bb1 unwind label %catch.dispatch0
@ -638,7 +687,7 @@ try.cont: ; preds = %catch.start1, %catc
; NOOPT: call foo
; NOOPT: end_block
; NOOPT: return
define void @test9(i32 %arg) personality i8* bitcast (i32 (...)* @__gxx_wasm_personality_v0 to i8*) {
define void @test10(i32 %arg) personality i8* bitcast (i32 (...)* @__gxx_wasm_personality_v0 to i8*) {
entry:
%tobool = icmp ne i32 %arg, 0
br i1 %tobool, label %if.then, label %if.end
@ -675,7 +724,7 @@ if.end: ; preds = %cont, %catch.start,
; invoke.cont BB fall within try~end_try, but they shouldn't cause crashes or
; unwinding destination mismatches in CFGStackify.
; NOSORT-LABEL: test10
; NOSORT-LABEL: test11
; NOSORT: try
; NOSORT: call foo
; NOSORT: call {{.*}} memcpy
@ -685,7 +734,7 @@ if.end: ; preds = %cont, %catch.start,
; NOSORT: catch
; NOSORT: rethrow
; NOSORT: end_try
define void @test10(i8* %a, i8* %b) personality i8* bitcast (i32 (...)* @__gxx_wasm_personality_v0 to i8*) {
define void @test11(i8* %a, i8* %b) personality i8* bitcast (i32 (...)* @__gxx_wasm_personality_v0 to i8*) {
entry:
%o = alloca %class.Object, align 1
invoke void @foo()
@ -709,11 +758,11 @@ ehcleanup: ; preds = %entry
; 'nothrow_i32' and 'fun', because the return value of 'nothrow_i32' is
; stackified and pushed onto the stack to be consumed by the call to 'fun'.
; CHECK-LABEL: test11
; CHECK-LABEL: test12
; CHECK: try
; CHECK: call $push{{.*}}=, nothrow_i32
; CHECK: call fun, $pop{{.*}}
define void @test11() personality i8* bitcast (i32 (...)* @__gxx_wasm_personality_v0 to i8*) {
define void @test12() personality i8* bitcast (i32 (...)* @__gxx_wasm_personality_v0 to i8*) {
entry:
%call = call i32 @nothrow_i32()
invoke void @fun(i32 %call)
@ -734,7 +783,7 @@ terminate: ; preds = %entry
; This crashed on debug mode (= when NDEBUG is not defined) when the logic for
; computing the innermost region was not correct, in which a loop region
; contains an exception region. This should pass CFGSort without crashing.
define void @test12() personality i8* bitcast (i32 (...)* @__gxx_wasm_personality_v0 to i8*) {
define void @test13() personality i8* bitcast (i32 (...)* @__gxx_wasm_personality_v0 to i8*) {
entry:
%e = alloca %class.MyClass, align 4
br label %for.cond
@ -802,11 +851,12 @@ terminate7: ; preds = %ehcleanup
}
; Check if the unwind destination mismatch stats are correct
; NOSORT-STAT: 14 wasm-cfg-stackify - Number of EH pad unwind mismatches found
; NOSORT-STAT: 15 wasm-cfg-stackify - Number of EH pad unwind mismatches found
declare void @foo()
declare void @bar()
declare i32 @baz()
declare i32 @qux(i32)
declare void @quux(i32)
declare void @fun(i32)
; Function Attrs: nounwind