BREAKING CHANGE: Do not allocate buffers for memory.data(i32, i32?)

MemorySegments can now hold null buffers, which signifies that memory is
in fact used in the compiled program, for programs that only reserve
zero-filled memories. Using null buffers avoids passing zero-filled
buffers to Binaryen, which isn't necessary since Wasm memories are
initialized with zeroes when a (data ...) segment isn't present.

This change is breaking in that modules using imported memories that
contain non-zero values in such memory ranges at the time of
instantiation will retain those values, as opposed to being filled.
This shouldn't affect most users, however, and it could possibly be
desirable.

Fixes #2827.

Author: CountBleck

src/builtins.ts

@@ -3322,7 +3322,7 @@ function builtin_memory_data(ctx: BuiltinFunctionContext): ExpressionRef {
         return module.unreachable();
       }
     }
-    offset = compiler.addAlignedMemorySegment(new Uint8Array(size), align).offset;
+    offset = compiler.addZeroedMemorySegment(size, align).offset;
   }
   // FIXME: what if recompiles happen? recompiles are bad.
   compiler.currentType = usizeType;

src/compiler.ts

@@ -1894,6 +1894,16 @@ export class Compiler extends DiagnosticEmitter {
     return segment;
   }
 
+  /** Adds a dummy memory segment. */
+  addZeroedMemorySegment(size: i32, alignment: i32 = 16): MemorySegment {
+    assert(isPowerOf2(alignment));
+    let memoryOffset = i64_align(this.memoryOffset, alignment);
+    let segment = new MemorySegment(null, memoryOffset);
+    this.memorySegments.push(segment);
+    this.memoryOffset = i64_add(memoryOffset, i64_new(size));
+    return segment;
+  }
+
   /** Adds a static memory segment representing a runtime object. */
   addRuntimeMemorySegment(buffer: Uint8Array): MemorySegment {
     let memoryOffset = this.program.computeBlockStart64(this.memoryOffset);
@@ -2043,7 +2053,7 @@ export class Compiler extends DiagnosticEmitter {
     if (!arrayInstance) {
       arrayInstance = assert(this.resolver.resolveClass(this.program.arrayPrototype, [ elementType ]));
     }
-    let bufferLength = readI32(bufferSegment.buffer, program.OBJECTInstance.offsetof("rtSize"));
+    let bufferLength = readI32(bufferSegment.buffer!, program.OBJECTInstance.offsetof("rtSize"));
     let arrayLength = i32(bufferLength / elementType.byteSize);
     let bufferAddress = i64_add(bufferSegment.offset, i64_new(program.totalOverhead));
     let buf = arrayInstance.createBuffer();
@@ -8060,7 +8070,7 @@ export class Compiler extends DiagnosticEmitter {
       }
       let arrayInstance = assert(this.resolver.resolveClass(this.program.staticArrayPrototype, [ stringType ]));
       let segment = this.addStaticBuffer(stringType, values, arrayInstance.id);
-      this.program.OBJECTInstance.writeField("gcInfo", 3, segment.buffer, 0); // use transparent gcinfo
+      this.program.OBJECTInstance.writeField("gcInfo", 3, segment.buffer!, 0); // use transparent gcinfo
       let offset = i64_add(segment.offset, i64_new(this.program.totalOverhead));
       let joinInstance = assert(arrayInstance.getMethod("join"));
       let indexedSetInstance = assert(arrayInstance.lookupOverload(OperatorKind.IndexedSet, true));
@@ -8151,7 +8161,7 @@ export class Compiler extends DiagnosticEmitter {
       );
       arrayInstance.writeField("raw",
         i64_add(rawHeaderSegment.offset, i64_new(this.program.totalOverhead)),
-        arraySegment.buffer
+        arraySegment.buffer!
       );
     } else {
       arraySegment = this.addStaticArrayHeader(stringType,