interp: align created globals

Use the alignment from the align attribute of the runtime.alloc call.
This is going to be a more accurate alignment, and is typically smaller
than the default.

builder/sizes_test.go

@@ -43,7 +43,7 @@ func TestBinarySize(t *testing.T) {
 		// microcontrollers
 		{"hifive1b", "examples/echo", 4600, 280, 0, 2268},
 		{"microbit", "examples/serial", 2908, 388, 8, 2272},
-		{"wioterminal", "examples/pininterrupt", 6140, 1484, 116, 6832},
+		{"wioterminal", "examples/pininterrupt", 6140, 1484, 116, 6824},
 
 		// TODO: also check wasm. Right now this is difficult, because
 		// wasm binaries are run through wasm-opt and therefore the

interp/interpreter.go

@@ -287,9 +287,17 @@ func (r *runner) run(fn *function, params []value, parentMem *memoryView, indent
 				// Get the object layout, if it is available.
 				llvmLayoutType := r.getLLVMTypeFromLayout(operands[2])
 
+				// Get the alignment of the memory to be allocated.
+				alignment := 0 // use default alignment if unset
+				alignAttr := inst.llvmInst.GetCallSiteEnumAttribute(0, llvm.AttributeKindID("align"))
+				if !alignAttr.IsNil() {
+					alignment = int(alignAttr.GetEnumValue())
+				}
+
 				// Create the object.
 				alloc := object{
 					globalName:     r.pkgName + "$alloc",
+					align:          alignment,
 					llvmLayoutType: llvmLayoutType,
 					buffer:         newRawValue(uint32(size)),
 					size:           uint32(size),
@@ -646,6 +654,7 @@ func (r *runner) run(fn *function, params []value, parentMem *memoryView, indent
 				globalName: r.pkgName + "$alloca",
 				buffer:     newRawValue(uint32(size)),
 				size:       uint32(size),
+				align:      inst.llvmInst.Alignment(),
 			}
 			index := len(r.objects)
 			r.objects = append(r.objects, alloca)

interp/memory.go

@@ -42,6 +42,7 @@ type object struct {
 	globalName     string    // name, if not yet created (not guaranteed to be the final name)
 	buffer         value     // buffer with value as given by interp, nil if external
 	size           uint32    // must match buffer.len(), if available
+	align          int       // alignment of the object (may be 0 if unknown)
 	constant       bool      // true if this is a constant global
 	marked         uint8     // 0 means unmarked, 1 means external read, 2 means external write
 }
@@ -593,6 +594,12 @@ func (v pointerValue) toLLVMValue(llvmType llvm.Type, mem *memoryView) (llvm.Val
 		// runtime.alloc.
 		// First allocate a new global for this object.
 		obj := mem.get(v.index())
+		alignment := obj.align
+		if alignment == 0 {
+			// Unknown alignment, perhaps from a direct call to runtime.alloc in
+			// the runtime. Use a conservative default instead.
+			alignment = mem.r.maxAlign
+		}
 		if obj.llvmType.IsNil() && obj.llvmLayoutType.IsNil() {
 			// Create an initializer without knowing the global type.
 			// This is probably the result of a runtime.alloc call.
@@ -603,7 +610,7 @@ func (v pointerValue) toLLVMValue(llvmType llvm.Type, mem *memoryView) (llvm.Val
 			globalType := initializer.Type()
 			llvmValue = llvm.AddGlobal(mem.r.mod, globalType, obj.globalName)
 			llvmValue.SetInitializer(initializer)
-			llvmValue.SetAlignment(mem.r.maxAlign)
+			llvmValue.SetAlignment(alignment)
 			obj.llvmGlobal = llvmValue
 			mem.put(v.index(), obj)
 		} else {
@@ -642,11 +649,7 @@ func (v pointerValue) toLLVMValue(llvmType llvm.Type, mem *memoryView) (llvm.Val
 				return llvm.Value{}, errors.New("interp: allocated value does not match allocated type")
 			}
 			llvmValue.SetInitializer(initializer)
-			if obj.llvmType.IsNil() {
-				// The exact type isn't known (only the layout), so use the
-				// alignment that would normally be expected from runtime.alloc.
-				llvmValue.SetAlignment(mem.r.maxAlign)
-			}
+			llvmValue.SetAlignment(alignment)
 		}
 
 		// It should be included in r.globals because otherwise markExternal