[WebAssembly] Handle multiple EH_LABELs in EH pad

[aheejin]

Summary

Usually EH_LABELs are placed in

• Before an invoke (which becomes calls in the backend)
• After an invoke
• At the start of an EH pad

I don't know exactly why, but I noticed there are cases of multiple, not
a single, EH_LABEL instructions in the beginning of an EH pad. In that
case global.set instruction placed to restore __stack_pointer ended
up between two EH_LABEL instructions before CATCH. It should follow
after the EH_LABELs and CATCH. This CL fixes that case.

llvm/lib/Target/WebAssembly/WebAssemblyLateEHPrepare.cpp

bool WebAssemblyLateEHPrepare::addCatchAlls(MachineFunction &MF) {
   for (auto &MBB : MF) {
     if (!MBB.isEHPad())
       continue;
     auto InsertPos = MBB.begin();
     // Skip EH_LABELs in the beginning of an EH pad if present.
     while (InsertPos != MBB.end() && InsertPos->isEHLabel())
       InsertPos++;
     // This runs after hoistCatches(), so we assume that if there is a catch,
-    // that should be the non-EH label first instruction in an EH pad.
+    // that should be the first non-EH-label instruction in an EH pad.
     if (InsertPos == MBB.end() ||
         !WebAssembly::isCatch(InsertPos->getOpcode())) {
       Changed = true;
       BuildMI(MBB, InsertPos, InsertPos->getDebugLoc(),
               TII.get(WebAssembly::CATCH_ALL));
     }
   }
   return Changed;
for (auto &MBB : MF) {
 
     // Insert __stack_pointer restoring instructions at the beginning of each EH
     // pad, after the catch instruction. Here it is safe to assume that SP32
     // holds the latest value of __stack_pointer, because the only exception for
     // this case is when a function uses the red zone, but that only happens
     // with leaf functions, and we don't restore __stack_pointer in leaf
     // functions anyway.
     auto InsertPos = MBB.begin();
-    if (InsertPos->isEHLabel()) // EH pad starts with an EH label
-      ++InsertPos;
-    if (WebAssembly::isCatch(InsertPos->getOpcode()))
-      ++InsertPos;
+    // Skip EH_LABELs in the beginning of an EH pad if present.
+    while (InsertPos != MBB.end() && InsertPos->isEHLabel())
+      InsertPos++;
+    assert(InsertPos != MBB.end() &&
+           WebAssembly::isCatch(InsertPos->getOpcode()) &&
+           "catch/catch_all should be present in every EH pad at this point");
+    ++InsertPos; // Skip the catch instruction
     FrameLowering->writeSPToGlobal(FrameLowering->getSPReg(MF), MF, MBB,
                                    InsertPos, MBB.begin()->getDebugLoc());
   }
   return Changed;
 }

llvm/test/CodeGen/WebAssembly/exception.mir

 
 ---
 # This tests 'try' and 'catch' instructions are correctly placed with respect to
 # EH_LABEL instructions.
 # CHECK-LABEL: name: eh_label_test
 name: eh_label_test
 liveins:
   - { reg: '$arguments' }
+frameInfo:
+  hasCalls: true
 body: |
   bb.0:
     ; TRY should be before EH_LABEL wrappers of throwing calls
     ; CHECK:      bb.0
     ; CHECK:      TRY
     ; CHECK-NEXT: EH_LABEL
     ; CHECK-NEXT: CALL @foo
     ; CHECK-NEXT: EH_LABEL
     successors: %bb.1, %bb.2
     EH_LABEL <mcsymbol .Ltmp0>
     CALL @foo, implicit-def dead $arguments, implicit $sp32, implicit $sp64
     EH_LABEL <mcsymbol .Ltmp1>
     BR %bb.2, implicit-def dead $arguments
 
   bb.1 (landing-pad):
   ; predecessors: %bb.0
     successors: %bb.2
-    ; CATCH_ALL should be after an EH_LABEL at the beginning of an EH pad
+    ; CATCH_ALL should be after EH_LABELs in the beginning of an EH pad.
+    ; (Sometimes there are multiple EH_LABELs in an EH pad. This test tests
+    ; that.) GLOBAL_SET should follow right after that.
     ; CHECK:      bb.1
     ; CHECK:      EH_LABEL
+    ; CHECK:      EH_LABEL
     ; CHECK-NEXT: CATCH_ALL
+    ; CHECK-NEXT: GLOBAL_SET_I32
+    EH_LABEL <mcsymbol .Ltmp2>
     EH_LABEL <mcsymbol .Ltmp2>
     CATCHRET %bb.2, %bb.1, implicit-def dead $arguments
 
   bb.2:
   ; predecessors: %bb.0, %bb.1
     RETURN implicit-def dead $arguments
 ...