compiler: add 'align' attribute to runtime.alloc calls

This adds something like 'align 4' to the runtime.alloc calls, so that
the compiler knows the alignment of the allocation and can optimize
accordingly.

The optimization is very small, only 0.01% in my small test. However, my
plan is to read the value in the heap-to-stack transform pass to make it
more correct and let it produce slightly more optimal code.

compiler/compiler.go

@@ -2017,6 +2017,8 @@ func (b *builder) createExpr(expr ssa.Value) (llvm.Value, error) {
 			sizeValue := llvm.ConstInt(b.uintptrType, size, false)
 			layoutValue := b.createObjectLayout(typ, expr.Pos())
 			buf := b.createRuntimeCall("alloc", []llvm.Value{sizeValue, layoutValue}, expr.Comment)
+			align := b.targetData.ABITypeAlignment(typ)
+			buf.AddCallSiteAttribute(0, b.ctx.CreateEnumAttribute(llvm.AttributeKindID("align"), uint64(align)))
 			return buf, nil
 		} else {
 			buf := llvmutil.CreateEntryBlockAlloca(b.Builder, typ, expr.Comment)
@@ -2223,6 +2225,7 @@ func (b *builder) createExpr(expr ssa.Value) (llvm.Value, error) {
 		sliceType := expr.Type().Underlying().(*types.Slice)
 		llvmElemType := b.getLLVMType(sliceType.Elem())
 		elemSize := b.targetData.TypeAllocSize(llvmElemType)
+		elemAlign := b.targetData.ABITypeAlignment(llvmElemType)
 		elemSizeValue := llvm.ConstInt(b.uintptrType, elemSize, false)
 
 		maxSize := b.maxSliceSize(llvmElemType)
@@ -2246,6 +2249,7 @@ func (b *builder) createExpr(expr ssa.Value) (llvm.Value, error) {
 		sliceSize := b.CreateBinOp(llvm.Mul, elemSizeValue, sliceCapCast, "makeslice.cap")
 		layoutValue := b.createObjectLayout(llvmElemType, expr.Pos())
 		slicePtr := b.createRuntimeCall("alloc", []llvm.Value{sliceSize, layoutValue}, "makeslice.buf")
+		slicePtr.AddCallSiteAttribute(0, b.ctx.CreateEnumAttribute(llvm.AttributeKindID("align"), uint64(elemAlign)))
 
 		// Extend or truncate if necessary. This is safe as we've already done
 		// the bounds check.