Performance considerations

Overall expectations

All in all, there should not be significant performance differences between code written in Ada and code written in C, provided that they are semantically equivalent. Taking the current GNAT implementation and its GCC C counterpart for example, most of the code generation and optimization phases are shared between C and Ada — so there's not one compiler more efficient than the other. Furthermore, the two languages are fairly similar in the way they implement imperative semantics, in particular with regards to memory management or control flow. They should be equivalent on average.

When comparing the performance of C and Ada code, differences might be observed. This usually comes from the fact that, while the two piece appear semantically equivalent, they happen to be actually quite different; C code semantics do not implicitly apply the same run-time checks that Ada does. This section will present common ways for improving Ada code performance.

Switches and optimizations

Clever use of compilation switches might optimize the performance of an application significantly. In this section, we'll briefly look into some of the switches available in the GNAT toolchain.

Optimizations levels

Optimization levels can be found in many compilers for multiple languages. On the lowest level, the GNAT compiler doesn't optimize the code at all, while at the higher levels, the compiler analyses the code and optimizes it by removing unnecessary operations and making the most use of the target processor's capabilities.

By being part of GCC, GNAT offers the same -O_ switches as GCC:

Switch

Description

-O0

No optimization: the generated code is completely unoptimized. This is the default optimization level.

-O1

Moderate optimization.

-O2

Full optimization.

-O3

Same optimization level as for -O2. In addition, further optimization strategies, such as aggressive automatic inlining and vectorization.

Note that the higher the level, the longer the compilation time. For fast compilation during development phase, unless you're working on benchmarking algorithms, using -O0 is probably a good idea.

In addition to the levels presented above, GNAT also has the -Os switch, which allows for optimizing code and data usage.

Inlining

As we've seen in the previous section, automatic inlining depends on the optimization level. The highest optimization level (-O3), for example, performs aggressive automatic inlining. This could mean that this level inlines too much rather than not enough. As a result, the cache may become an issue and the overall performance may be worse than the one we would achieve by compiling the same code with optimization level 2 (-O2). Therefore, the general recommendation is to not just select -O3 for the optimized version of an application, but instead compare it the optimized version built with -O2.

In some cases, it's better to reduce the optimization level and perform manual inlining instead of automatic inlining. We do that by using the Inline aspect. Let's reuse an example from a previous chapter and inline the Average function:

[Ada]

package Float_Arrays is type Float_Array is array (Positive range <>) of Float; function Average (Data : Float_Array) return Float with Inline; end Float_Arrays;
package body Float_Arrays is function Average (Data : Float_Array) return Float is Total : Float := 0.0; begin for Value of Data loop Total := Total + Value; end loop; return Total / Float (Data'Length); end Average; end Float_Arrays;
with Ada.Text_IO; use Ada.Text_IO; with Float_Arrays; use Float_Arrays; procedure Compute_Average is Values : constant Float_Array := (10.0, 11.0, 12.0, 13.0); Average_Value : Float; begin Average_Value := Average (Values); Put_Line ("Average = " & Float'Image (Average_Value)); end Compute_Average;

When compiling this example, GNAT will inline Average in the Compute_Average procedure.

In order to effectively use this aspect, however, we need to set the optimization level to at least -O1 and use the -gnatn switch, which instructs the compiler to take the Inline aspect into account.

Note, however, that the Inline aspect is just a recommendation to the compiler. Sometimes, the compiler might not be able to follow this recommendation, so it won't inline the subprogram. In this case, we get a compilation warning from GNAT.

These are some examples of situations where the compiler might not be able to inline a subprogram:

  • when the code is too large,

  • when it's too complicated — for example, when it involves exception handling —, or

  • when it contains tasks, etc.

In addition to the Inline aspect, we also have the Inline_Always aspect. In contrast to the former aspect, however, the Inline_Always aspect isn't primarily related to performance. Instead, it should be used when the functionality would be incorrect if inlining was not performed by the compiler. Examples of this are procedures that insert Assembly instructions that only make sense when the procedure is inlined, such as memory barriers.

Similar to the Inline aspect, there might be situations where a subprogram has the Inline_Always aspect, but the compiler is unable to inline it. In this case, we get a compilation error from GNAT.

Checks and assertions

Checks

Ada provides many runtime checks to ensure that the implementation is working as expected. For example, when accessing an array, we would like to make sure that we're not accessing a memory position that is not allocated for that array. This is achieved by an index check.

Another example of runtime check is the verification of valid ranges. For example, when adding two integer numbers, we would like to ensure that the result is still in the valid range — that the value is neither too large nor too small. This is achieved by an range check. Likewise, arithmetic operations shouldn't overflow or underflow. This is achieved by an overflow check.

Although runtime checks are very useful and should be used as much as possible, they can also increase the overhead of implementations at certain hot-spots. For example, checking the index of an array in a sorting algorithm may significantly decrease its performance. In those cases, suppressing the check may be an option. We can achieve this suppression by using pragma Suppress (Index_Check). For example:

[Ada]

procedure Sort (A : in out Integer_Array) is
   pragma Suppress (Index_Check);
begin
   --  (implementation removed...)
   null;
end Sort;

In case of overflow checks, we can use pragma Suppress (Overflow_Check) to suppress them:

function Some_Computation (A, B : Int32) return Int32 is
   pragma Suppress (Overflow_Check);
begin
   --  (implementation removed...)
   null;
end Sort;

We can also deactivate overflow checks for integer types using the -gnato switch when compiling a source-code file with GNAT. In this case, overflow checks in the whole file are deactivated.

It is also possible to suppress all checks at once using pragma Suppress (All_Checks). In addition, GNAT offers a compilation switch called -gnatp, which has the same effect on the whole file.

Note, however, that this kind of suppression is just a recommendation to the compiler. There's no guarantee that the compiler will actually suppress any of the checks because the compiler may not be able to do so — typically because the hardware happens to do it. For example, if the machine traps on any access via address zero, requesting the removal of null access value checks in the generated code won't prevent the checks from happening.

It is important to differentiate between required and redundant checks. Let's consider the following example in C:

[C]

#include <stdio.h> int main(int argc, const char * argv[]) { int a = 8, b = 0, res; res = a / b; // printing the result printf("res = %d\n", res); return 0; }

Because C doesn't have language-defined checks, as soon as the application tries to divide a value by zero in res = a / b, it'll break — on Linux, for example, you may get the following error message by the operating system: Floating point exception (core dumped). Therefore, we need to manually introduce a check for zero before this operation. For example:

[C]

#include <stdio.h> int main(int argc, const char * argv[]) { int a = 8, b = 0, res; if (b != 0) { res = a / b; // printing the result printf("res = %d\n", res); } else { // printing error message printf("Error: cannot calculate value (division by zero)\n"); } return 0; }

This is the corresponding code in Ada:

[Ada]

with Ada.Text_IO; use Ada.Text_IO; procedure Show_Division_By_Zero is A : Integer := 8; B : Integer := 0; Res : Integer; begin Res := A / B; Put_Line ("Res = " & Integer'Image (Res)); end Show_Division_By_Zero;

Similar to the first version of the C code, we're not explicitly checking for a potential division by zero here. In Ada, however, this check is automatically inserted by the language itself. When running the application above, an exception is raised when the application tries to divide the value in A by zero. We could introduce exception handling in our example, so that we get the same message as we did in the second version of the C code:

[Ada]

with Ada.Text_IO; use Ada.Text_IO; procedure Show_Division_By_Zero is A : Integer := 8; B : Integer := 0; Res : Integer; begin Res := A / B; Put_Line ("Res = " & Integer'Image (Res)); exception when Constraint_Error => Put_Line ("Error: cannot calculate value (division by zero)"); when others => null; end Show_Division_By_Zero;

This example demonstrates that the division check for Res := A / B is required and shouldn't be suppressed. In contrast, a check is redundant — and therefore not required — when we know that the condition that leads to a failure can never happen. In many cases, the compiler itself detects redundant checks and eliminates them (for higher optimization levels). Therefore, when improving the performance of your application, you should:

  1. keep all checks active for most parts of the application;

  2. identify the hot-spots of your application;

  3. identify which checks haven't been eliminated by the optimizer on these hot-spots;

  4. identify which of those checks are redundant;

  5. only suppress those checks that are redundant, and keep the required ones.

Assertions

We've already discussed assertions in this section of the SPARK chapter. Assertions are user-defined checks that you can add to your code using the pragma Assert. For example:

[Ada]

function Some_Computation (A, B : Int32) return Int32 is
   Res : Int32;
begin
   --  (implementation removed...)

   pragma Assert (Res >= 0);

   return Res;
end Sort;

Assertions that are specified with pragma Assert are not enabled by default. You can enable them by setting the assertion policy to check — using pragma Assertion_Policy (Check) — or by using the -gnata switch when compiling with GNAT.

Similar to the checks discussed previously, assertions can generate significant overhead when used at hot-spots. Restricting those assertions to development (e.g. debug version) and turning them off on the release version may be an option. In this case, formal proof — as discussed in the SPARK chapter — can help you. By formally proving that assertions will never fail at run-time, you can safely deactivate them.

Dynamic vs. static structures

Ada generally speaking provides more ways than C or C++ to write simple dynamic structures, that is to say structures that have constraints computed after variables. For example, it's quite typical to have initial values in record types:

[Ada]

type R is record
   F : Some_Field := Call_To_Some_Function;
end record;

However, the consequences of the above is that any declaration of a instance of this type without an explicit value for V will issue a call to Call_To_Some_Function. More subtle issue may arise with elaboration. For example, it's possible to write:

package Some_Functions is function Some_Function_Call return Integer is (2); function Some_Other_Function_Call return Integer is (10); end Some_Functions;
with Some_Functions; use Some_Functions; package Values is A_Start : Integer := Some_Function_Call; A_End : Integer := Some_Other_Function_Call; end Values;
with Values; use Values; package Arr_Def is type Arr is array (Integer range A_Start .. A_End) of Integer; end Arr_Def;

It may indeed be appealing to be able to change the values of A_Start and A_End at startup so as to align a series of arrays dynamically. The consequence, however, is that these values will not be known statically, so any code that needs to access to boundaries of the array will need to read data from memory. While it's perfectly fine most of the time, there may be situations where performances are so critical that static values for array boundaries must be enforced.

Here's a last case which may also be surprising:

[Ada]

package Arr_Def is type Arr is array (Integer range <>) of Integer; type R (D1, D2 : Integer) is record F1 : Arr (1 .. D1); F2 : Arr (1 .. D2); end record; end Arr_Def;

In the code above, R contains two arrays, F1 and F2, respectively constrained by the discriminant D1 and D2. The consequence is, however, that to access F2, the run-time needs to know how large F1 is, which is dynamically constrained when creating an instance. Therefore, accessing to F2 requires a computation involving D1 which is slower than, let's say, two pointers in an C array that would point to two different arrays.

Generally speaking, when values are used in data structures, it's useful to always consider where they're coming from, and if their value is static (computed by the compiler) or dynamic (only known at run-time). There's nothing fundamentally wrong with dynamically constrained types, unless they appear in performance-critical pieces of the application.

Pointers vs. data copies

In the section about pointers, we mentioned that the Ada compiler will automatically pass parameters by reference when needed. Let's look into what "when needed" means. The fundamental point to understand is that the parameter types determine how the parameters are passed in and/or out. The parameter modes do not control how parameters are passed.

Specifically, the language standards specifies that scalar types are always passed by value, and that some other types are always passed by reference. It would not make sense to make a copy of a task when passing it as a parameter, for example. So parameters that can be passed reasonably by value will be, and those that must be passed by reference will be. That's the safest approach.

But the language also specifies that when the parameter is an array type or a record type, and the record/array components are all by-value types, then the compiler decides: it can pass the parameter using either mechanism. The critical case is when such a parameter is large, e.g., a large matrix. We don't want the compiler to pass it by value because that would entail a large copy, and indeed the compiler will not do so. But if the array or record parameter is small, say the same size as an address, then it doesn't matter how it is passed and by copy is just as fast as by reference. That's why the language gives the choice to the compiler. Although the language does not mandate that large parameters be passed by reference, any reasonable compiler will do the right thing.

The modes do have an effect, but not in determining how the parameters are passed. Their effect, for parameters passed by value, is to determine how many times the value is copied. For mode in and mode out there is just one copy. For mode in out there will be two copies, one in each direction.

Therefore, unlike C, you don't have to use access types in Ada to get better performance when passing arrays or records to subprograms. The compiler will almost certainly do the right thing for you.

Let's look at this example:

[C]

#include <stdio.h> struct Data { int prev, curr; }; void update(struct Data *d, int v) { d->prev = d->curr; d->curr = v; } void display(const struct Data *d) { printf("Prev : %d\n", d->prev); printf("Curr : %d\n", d->curr); } int main(int argc, const char * argv[]) { struct Data D1 = { 0, 1 }; update (&D1, 3); display (&D1); return 0; }

In this C code example, we're using pointers to pass D1 as a reference to update and display. In contrast, the equivalent code in Ada simply uses the parameter modes to specify the data flow directions. The mechanisms used to pass the values do not appear in the source code.

[Ada]

with Ada.Text_IO; use Ada.Text_IO; procedure Update_Record is type Data is record Prev : Integer; Curr : Integer; end record; procedure Update (D : in out Data; V : Integer) is begin D.Prev := D.Curr; D.Curr := V; end Update; procedure Display (D : Data) is begin Put_Line ("Prev: " & Integer'Image (D.Prev)); Put_Line ("Curr: " & Integer'Image (D.Curr)); end Display; D1 : Data := (0, 1); begin Update (D1, 3); Display (D1); end Update_Record;

In the calls to Update and Display, D1 is always be passed by reference. Because no extra copy takes place, we get a performance that is equivalent to the C version. If we had used arrays in the example above, D1 would have been passed by reference as well:

[Ada]

with Ada.Text_IO; use Ada.Text_IO; procedure Update_Array is type Data_State is (Prev, Curr); type Data is array (Data_State) of Integer; procedure Update (D : in out Data; V : Integer) is begin D (Prev) := D (Curr); D (Curr) := V; end Update; procedure Display (D : Data) is begin Put_Line ("Prev: " & Integer'Image (D (Prev))); Put_Line ("Curr: " & Integer'Image (D (Curr))); end Display; D1 : Data := (0, 1); begin Update (D1, 3); Display (D1); end Update_Array;

Again, no extra copy is performed in the calls to Update and Display, which gives us optimal performance when dealing with arrays and avoids the need to use access types to optimize the code.

Function returns

Previously, we've discussed the cost of passing complex records as arguments to subprograms. We've seen that we don't have to use explicit access type parameters to get better performance in Ada. In this section, we'll briefly discuss the cost of function returns.

In general, we can use either procedures or functions to initialize a data structure. Let's look at this example in C:

[C]

#include <stdio.h> struct Data { int prev, curr; }; void init_data(struct Data *d) { d->prev = 0; d->curr = 1; } struct Data get_init_data() { struct Data d = { 0, 1 }; return d; } int main(int argc, const char * argv[]) { struct Data D1; D1 = get_init_data(); init_data(&D1); return 0; }

This code example contains two subprograms that initialize the Data structure:

  • init_data(), which receives the data structure as a reference (using a pointer) and initializes it, and

  • get_init_data(), which returns the initialized structure.

In C, we generally avoid implementing functions such as get_init_data() because of the extra copy that is needed for the function return.

This is the corresponding implementation in Ada:

[Ada]

procedure Init_Record is type Data is record Prev : Integer; Curr : Integer; end record; procedure Init (D : out Data) is begin D := (Prev => 0, Curr => 1); end Init; function Init return Data is D : constant Data := (Prev => 0, Curr => 1); begin return D; end Init; D1 : Data; pragma Unreferenced (D1); begin D1 := Init; Init (D1); end Init_Record;

In this example, we have two versions of Init: one using a procedural form, and the other one using a functional form. Note that, because of Ada's support for subprogram overloading, we can use the same name for both subprograms.

The issue is that assignment of a function result entails a copy, just as if we assigned one variable to another. For example, when assigning a function result to a constant, the function result is copied into the memory for the constant. That's what is happening in the above examples for the initialized variables.

Therefore, in terms of performance, the same recommendations apply: for large types we should avoid writing functions like the Init function above. Instead, we should use the procedural form of Init. The reason is that the compiler necessarily generates a copy for the Init function, while the Init procedure uses a reference for the output parameter, so that the actual record initialization is performed in place in the caller's argument.

An exception to this is when we use functions returning values of limited types, which by definition do not allow assignment. Here, to avoid allowing something that would otherwise look suspiciously like an assignment, the compiler generates the function body so that it builds the result directly into the object being assigned. No copy takes place.

We could, for example, rewrite the example above using limited types:

[Ada]

procedure Init_Limited_Record is type Data is limited record Prev : Integer; Curr : Integer; end record; function Init return Data is begin return D : Data do D.Prev := 0; D.Curr := 1; end return; end Init; D1 : Data := Init; pragma Unreferenced (D1); begin null; end Init_Limited_Record;

In this example, D1 : Data := Init; has the same cost as the call to the procedural form — Init (D1); — that we've seen in the previous example. This is because the assignment is done in place.

Note that limited types require the use of the extended return statements (return ... do ... end return) in function implementations. Also note that, because the Data type is limited, we can only use the Init function in the declaration of D1; a statement in the code such as D1 := Init; is therefore forbidden.