[llvm] f178f61 - [WebAssembly] Nullify unnecessary setjmp calls

[Heejin Ahn via llvm-commits]

Author: Heejin Ahn
Date: 2022-01-04T17:44:32-08:00
New Revision: f178f61e1dd29e2e97d608fa5191f93e50a1be63

URL: https://github.com/llvm/llvm-project/commit/f178f61e1dd29e2e97d608fa5191f93e50a1be63
DIFF: https://github.com/llvm/llvm-project/commit/f178f61e1dd29e2e97d608fa5191f93e50a1be63.diff

LOG: [WebAssembly] Nullify unnecessary setjmp calls

D107530 did a small optimization that, if a function contains `setjmp`
calls but not other calls that can `longjmp`, we don't do SjLj
transformation on those `setjmp` calls, because they don't have
possibilities of returning from `longjmp`.

But we should remove those `setjmp` calls even in that case, because
Emscripten doesn't provide that function, assuming it is lowered away by
SjLj transformation. `setjmp` always returns 0 when called directly, so
this CL replaces them with `i32 0`.

Fixes https://github.com/emscripten-core/emscripten/issues/15679.

Reviewed By: dschuff

Differential Revision: https://reviews.llvm.org/D116619

Added: 
    

Modified: 
    llvm/lib/Target/WebAssembly/WebAssemblyLowerEmscriptenEHSjLj.cpp
    llvm/test/CodeGen/WebAssembly/lower-em-sjlj.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Target/WebAssembly/WebAssemblyLowerEmscriptenEHSjLj.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyLowerEmscriptenEHSjLj.cpp
index 23aaa5160abd2..df96c6f437c2e 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyLowerEmscriptenEHSjLj.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyLowerEmscriptenEHSjLj.cpp
@@ -817,6 +817,32 @@ static bool containsLongjmpableCalls(const Function *F) {
   return false;
 }
 
+// When a function contains a setjmp call but not other calls that can longjmp,
+// we don't do setjmp transformation for that setjmp. But we need to convert the
+// setjmp calls into "i32 0" so they don't cause link time errors. setjmp always
+// returns 0 when called directly.
+static void nullifySetjmp(Function *F) {
+  Module &M = *F->getParent();
+  IRBuilder<> IRB(M.getContext());
+  Function *SetjmpF = M.getFunction("setjmp");
+  SmallVector<Instruction *, 1> ToErase;
+
+  for (User *U : SetjmpF->users()) {
+    auto *CI = dyn_cast<CallInst>(U);
+    // FIXME 'invoke' to setjmp can happen when we use Wasm EH + Wasm SjLj, but
+    // we don't support two being used together yet.
+    if (!CI)
+      report_fatal_error("Wasm EH + Wasm SjLj is not fully supported yet");
+    BasicBlock *BB = CI->getParent();
+    if (BB->getParent() != F) // in other function
+      continue;
+    ToErase.push_back(CI);
+    CI->replaceAllUsesWith(IRB.getInt32(0));
+  }
+  for (auto *I : ToErase)
+    I->eraseFromParent();
+}
+
 bool WebAssemblyLowerEmscriptenEHSjLj::runOnModule(Module &M) {
   LLVM_DEBUG(dbgs() << "********** Lower Emscripten EH & SjLj **********\n");
 
@@ -886,6 +912,10 @@ bool WebAssemblyLowerEmscriptenEHSjLj::runOnModule(Module &M) {
     EHTypeIDF = getEmscriptenFunction(EHTypeIDTy, "llvm_eh_typeid_for", &M);
   }
 
+  // Functions that contains calls to setjmp but don't have other longjmpable
+  // calls within them.
+  SmallPtrSet<Function *, 4> SetjmpUsersToNullify;
+
   if ((EnableEmSjLj || EnableWasmSjLj) && SetjmpF) {
     // Precompute setjmp users
     for (User *U : SetjmpF->users()) {
@@ -896,6 +926,8 @@ bool WebAssemblyLowerEmscriptenEHSjLj::runOnModule(Module &M) {
         // so can ignore it
         if (containsLongjmpableCalls(UserF))
           SetjmpUsers.insert(UserF);
+        else
+          SetjmpUsersToNullify.insert(UserF);
       } else {
         std::string S;
         raw_string_ostream SS(S);
@@ -975,6 +1007,14 @@ bool WebAssemblyLowerEmscriptenEHSjLj::runOnModule(Module &M) {
         runSjLjOnFunction(*F);
   }
 
+  // Replace unnecessary setjmp calls with 0
+  if ((EnableEmSjLj || EnableWasmSjLj) && !SetjmpUsersToNullify.empty()) {
+    Changed = true;
+    assert(SetjmpF);
+    for (Function *F : SetjmpUsersToNullify)
+      nullifySetjmp(F);
+  }
+
   if (!Changed) {
     // Delete unused global variables and functions
     if (ResumeF)

diff  --git a/llvm/test/CodeGen/WebAssembly/lower-em-sjlj.ll b/llvm/test/CodeGen/WebAssembly/lower-em-sjlj.ll
index fcfd503234065..77dcff2e0799e 100644
--- a/llvm/test/CodeGen/WebAssembly/lower-em-sjlj.ll
+++ b/llvm/test/CodeGen/WebAssembly/lower-em-sjlj.ll
@@ -131,7 +131,7 @@ entry:
 
 ; Test a case where a function has a setjmp call but no other calls that can
 ; longjmp. We don't need to do any transformation in this case.
-define void @setjmp_only(i8* %ptr) {
+define i32 @setjmp_only(i8* %ptr) {
 ; CHECK-LABEL: @setjmp_only
 entry:
   %buf = alloca [1 x %struct.__jmp_buf_tag], align 16
@@ -139,11 +139,12 @@ entry:
   %call = call i32 @setjmp(%struct.__jmp_buf_tag* %arraydecay) #0
   ; free cannot longjmp
   call void @free(i8* %ptr)
-  ret void
+  ret i32 %call
 ; CHECK-NOT: @malloc
 ; CHECK-NOT: %setjmpTable
 ; CHECK-NOT: @saveSetjmp
 ; CHECK-NOT: @testSetjmp
+; CHECK: ret i32 0
 }
 
 ; Test SSA validity