HACK: Atomics.exchange() is not yet implemented, so as a temp workaround, do it as u32 version with a CAS loop. TODO: revert this commit once https://bugzilla.mozilla.org/show_bug.cgi?id=1141986 lands.

Author: juj

lib/Target/JSBackend/CallHandlers.h

@@ -546,6 +546,8 @@ std::string getAddressAsString(const Value *Ptr, int shift) {
   }
 }
 
+/* TODO: Uncomment once https://bugzilla.mozilla.org/show_bug.cgi?id=1141986 is implemented!
+
 DEF_CALL_HANDLER(emscripten_atomic_exchange_u8, {
   return getAssign(CI) + "Atomics_exchange(HEAP8, " + getValueAsStr(CI->getOperand(0)) + ", " + getValueAsStr(CI->getOperand(1)) + ")";
 })
@@ -561,6 +563,7 @@ DEF_CALL_HANDLER(emscripten_atomic_exchange_f32, {
 DEF_CALL_HANDLER(emscripten_atomic_exchange_f64, {
   return getAssign(CI) + "Atomics_exchange(HEAPF64, " + getAddressAsString(CI->getOperand(0), 3) + ", " + getValueAsStr(CI->getOperand(1)) + ")";
 })
+*/
 
 DEF_CALL_HANDLER(emscripten_atomic_cas_u8, {
   return getAssign(CI) + "Atomics_compareExchange(HEAP8, " + getValueAsStr(CI->getOperand(0)) + ", " + getValueAsStr(CI->getOperand(1)) + ", " + getValueAsStr(CI->getOperand(2)) + ")";
@@ -868,11 +871,13 @@ void setupCallHandlers() {
   SETUP_CALL_HANDLER(emscripten_asm_const_int);
   SETUP_CALL_HANDLER(emscripten_asm_const_double);
 
+/* TODO: Uncomment once https://bugzilla.mozilla.org/show_bug.cgi?id=1141986 is implemented!
   SETUP_CALL_HANDLER(emscripten_atomic_exchange_u8);
   SETUP_CALL_HANDLER(emscripten_atomic_exchange_u16);
   SETUP_CALL_HANDLER(emscripten_atomic_exchange_u32);
   SETUP_CALL_HANDLER(emscripten_atomic_exchange_f32);
   SETUP_CALL_HANDLER(emscripten_atomic_exchange_f64);
+*/
 
   SETUP_CALL_HANDLER(emscripten_atomic_cas_u8);
   SETUP_CALL_HANDLER(emscripten_atomic_cas_u16);

lib/Target/JSBackend/JSBackend.cpp

@@ -2385,6 +2385,11 @@ void JSWriter::generateExpression(const User *I, raw_string_ostream& Code) {
         // implemented, we could remove the emulation, but until then we must emulate manually.
         bool fround = PreciseF32 && !strcmp(HeapName, "HEAPF32");
         Code << Assign << (fround ? "Math_fround(" : "+") << "_emscripten_atomic_" << atomicFunc << "_" << heapNameToAtomicTypeName(HeapName) << "(" << getByteAddressAsStr(P) << ", " << VS << (fround ? "))" : ")"); break;
+
+      // TODO: Remove the following two lines once https://bugzilla.mozilla.org/show_bug.cgi?id=1141986 is implemented!
+      } else if (rmwi->getOperation() == AtomicRMWInst::Xchg && !strcmp(HeapName, "HEAP32")) {
+        Code << Assign << "_emscripten_atomic_exchange_u32(" << getByteAddressAsStr(P) << ", " << VS << ")|0"; break;
+
       } else {
         Code << Assign << "Atomics_" << atomicFunc << "(" << HeapName << ", " << Index << ", " << VS << ")"; break;
       }