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Date: Sat, 09 Jun 2012 09:04:14 +0200
From: Georg Bauhaus <rm.dash-bauhaus@futureapps.de>
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Subject: Re: Practicalities of Ada for app development
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Date: 2012-06-09T09:04:11+02:00
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On 08.06.12 22:48, wrp wrote:

> 6. Size and latency are sometimes an issue. I've heard that since GNAT
> is oriented to building larger systems, the executables it produces
> are comparatively bulky. What is the situation relative to C in that
> regard?

Diagnoses of relative sizes of executables have frequently neglected
the effect of static vs dynamic linking, for example.

For the following functions, written in C and Ada, GCC produces
sequences of processor instructions that are almost identical:

-- 8< --

#include <stdint.h>

typedef double Real;
typedef int32_t Nat;

static Nat Last = 42;

Real Gen_Random (const Real Max) {
#define IM 139968
#define IA   3877
#define IC  29573
   
   Last = (Last * IA + IC) % IM;
   return (Max * (Real) Last) / (Real) IM;
}

-- 8< --

package Nothing is
    subtype Real is Long_Float;

    function Gen_Random (Max : in Real) return Real;
end Nothing;

package body Nothing is
    Last : Natural := 42;

    function Gen_Random (Max : in Real) return Real is
       IM : constant := 139_968;
       IA : constant :=   3_877;
       IC : constant :=  29_573;
    begin
       Last := (Last * IA + IC) mod IM;
       return (Max * Real (Last)) / Real (IM);
    end Gen_Random;
end Nothing;

-- 8< --

Translations:
$ gcc -W -c -O nothing.c
$ otool -tv nothing.o  # C output

_Gen_Random:
0000000000000000	pushq	%rbp
0000000000000001	movq	%rsp,%rbp
0000000000000004	imull	$0x00000f25,0xfffffffc(%rip),%ecx
000000000000000e	addl	$0x00007385,%ecx
0000000000000014	movl	$0x1df75681,%edx
0000000000000019	movl	%ecx,%eax
000000000000001b	imull	%edx
000000000000001d	sarl	$0x0e,%edx
0000000000000020	movl	%ecx,%eax
0000000000000022	sarl	$0x1f,%eax
0000000000000025	subl	%eax,%edx
0000000000000027	imull	$0x000222c0,%edx,%edx
000000000000002d	subl	%edx,%ecx
000000000000002f	movl	%ecx,0x00000000(%rip)
0000000000000035	cvtsi2sd	%ecx,%xmm1
0000000000000039	mulsd	%xmm1,%xmm0
000000000000003d	divsd	0x00000023(%rip),%xmm0
0000000000000045	leave
0000000000000046	ret
0000000000000047	nop

$ gnatchop -r -w nothing.ada && gnatmake -gnatwa -gnatp -O nothing.adb
$ otool -tv nothing.o   # Ada output

_nothing__gen_random:
0000000000000000	pushq	%rbp
0000000000000001	movq	%rsp,%rbp
0000000000000004	imull	$0x00000f25,0xfffffffc(%rip),%ecx
000000000000000e	addl	$0x00007385,%ecx
0000000000000014	movl	%ecx,%esi
0000000000000016	sarl	$0x1f,%esi
0000000000000019	movl	%ecx,%eax
000000000000001b	xorl	%esi,%eax
000000000000001d	movl	$0x1df75681,%edx
0000000000000022	mull	%edx
0000000000000024	shrl	$0x0e,%edx
0000000000000027	xorl	%edx,%esi
0000000000000029	imull	$0x000222c0,%esi,%esi
000000000000002f	subl	%esi,%ecx
0000000000000031	movl	%ecx,0x00000000(%rip)
0000000000000037	cvtsi2sd	%ecx,%xmm1
000000000000003b	mulsd	%xmm1,%xmm0
000000000000003f	divsd	0x00000011(%rip),%xmm0
0000000000000047	leave
0000000000000048	ret

While not identical, and while the Ada option -gnatp suppresses all
checks in order to compete with C on a size scale, this ad hoc test
demonstrates the extent to which there is support for saying
an Ada executable will be "bulky", I think.