interrupt: fix bug in interrupt lowering

The alignment wasn't set, so defaulted to 4 (for a 32-bit int). LLVM saw
this, and therefore assumed that a ptrtoint of the pointer would have
had the lowest bits unset. That's an entirely valid optimization, except
that we are using these globals for arbitrary values (and aren't
actually using these globals).

Fixed by setting alignment to 1. It works, though long-term we should
maybe find a different solution for this.

compiler/interrupt.go

@@ -41,6 +41,8 @@ func (b *builder) createInterruptGlobal(instr *ssa.CallCommon) (llvm.Value, erro
 
 	// Create a new global of type runtime/interrupt.handle. Globals of this
 	// type are lowered in the interrupt lowering pass.
+	// It must have an alignment of 1, otherwise LLVM thinks a ptrtoint of the
+	// global has the lower bits unset.
 	globalType := b.program.ImportedPackage("runtime/interrupt").Type("handle").Type()
 	globalLLVMType := b.getLLVMType(globalType)
 	globalName := b.fn.Package().Pkg.Path() + "$interrupt" + strconv.FormatInt(id.Int64(), 10)
@@ -48,6 +50,7 @@ func (b *builder) createInterruptGlobal(instr *ssa.CallCommon) (llvm.Value, erro
 	global.SetVisibility(llvm.HiddenVisibility)
 	global.SetGlobalConstant(true)
 	global.SetUnnamedAddr(true)
+	global.SetAlignment(1)
 	initializer := llvm.ConstNull(globalLLVMType)
 	initializer = b.CreateInsertValue(initializer, funcContext, 0, "")
 	initializer = b.CreateInsertValue(initializer, funcPtr, 1, "")