File size and smartlinking

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This article is a work in progress about executable size and smartlinking on Lazarus. Feel free to contribute.


In Free Pascal, "smartlinking" is removing unused code and variables from the final executable. This is done during the linking stage, when the executable is written using the object files created by the compiler earlier.

Case study 1 in Windows

This study was conducted on the 8th of February 2006 because a Lazarus fully compiled with Smartlinking was Released (version 0.9.12). It intends to establish the relationship between the varying results below with different Lazarus and Free Pascal versions as well as with Smartlinking and without.

The Variables being studied are:

  • Executable size after strip
  • Executable size after strip and UPX
  • Linking time

Compile time isn´t considered here because it´s too similar on all configurations and much less significant than the link time.

Executable size without strip isn´t included. Notice that strip was used always from command line with the command:

strip --strip-all magnifier.exe

The program being compiled is the Virtual Magnifying Glass. The source and binaries for this program are freely available for download on:

About the linking time please note that the utilized computer is 3.2GHz Pentium 4 with Intel motherboard and dual core processor and 512MB of RAM.


The utilized OS is Windows XP and the 0.9.13 versions are from the same date when 0.9.12 was release. The comparison took place using the following software configurations:

  • Lazarus 0.9.12 available here. Free Pascal 2.0.2 that comes with the installer. LCL and RTL are smartlinked. Refered from now on as simply 0.9.12.
  • Lazarus 0.9.13 downloaded from Subversion from the same date. Free Pascal 2.0.2 installed separately. The LCL is not smartlinked. Refered from now on as simply 0.9.13 + 2.0.2.
  • Lazarus 0.9.13 snapshot. Free Pascal 2.1 that comes with the installer. The LCL is not smartlinked. Refered from now on as simply 0.9.13 + 2.1.
  • Lazarus 0.9.13 snapshot. Free Pascal 2.1 that comes with the installer. The LCL is smartlinked. Refered from now on as simply 0.9.13 + 2.1 + SL.
0.9.12 0.9.13 + 2.0.2 0.9.13 + 2.1 + SL 0.9.13 + 2.1
File Size after strip (in bytes) 1,108,480 1,587,712 1,425,408 1,649,152
File Size after UPX (in bytes) 318,976 438,272 388,608 454,144
Linking time 15 seconds 5 seconds 45 seconds 10 seconds



The 0.9.13 snapshot from the 8th of February 2006 features a unstable compiler from the 2.1 branch, which can cause the bigger executables and slower linking time as compared to the other versions.

The 0.9.12 version has the best file size of all, both with UPX and without, showing that Smartlinking really can diminish the file size in Windows. This, however, does not come without a cost, and the cost is linking time, which is about 3 times higher then without smartlinking.

The 0.9.12 version already comes fully configured for Smartlinking on Windows and no extra configuration is needed. This was not the case on previous releases.

Remove unused functions in a class

Q: If a virtual method of a class is not used at all in the programm will the compiler remove it from the executable ?

A: If you use FPC 2.3.1 with whole-program optimization and the compiler can prove that it is never callable: yes. See Whole Program Optimization for more information. In case this is in relation to the thread on the Lazarus list about the big executables: note that it has only a limited effect on Lazarus programs, because almost all linked LCL code can potentially be executed (due to the way the LCL is constructed). In fact, I think most savings there come from making a number of virtual method calls non-virtual, rather than from throwing away unreachable code.

The internal linker can also do it (only throwing away virtual method calls, not turning virtual method calls into static ones), but only on Windows platforms. It is however not currently enabled in the compiler, because the changes break the external linker. It should therefore be turned into a command line option (along with a check that produces an error if you try to link a unit compiled with the option using the external linker), but that hasn't been done yet.

Q: How can the compiler determine that a virtual method is unused at all,

A: If you have a class hierarchy TBase->TDerived1->TDerived2 with a virtual method called "vmethod", and nowhere in the program there is a call to vmethod, then it is unused. Or if it is only called using TDerived2 instances, then if the linker does not find any direct references to TDerived1.vmethod or TBase.vmethod (e.g., via "inherited" calls from TDerived2 methods), it knows that the VMT entries for "vmethod" in TDerived1 and TBase can be set to nil.

Bytes Used

Here's a layout by pascal units in bytes used for code. The size is based on Dwarf2 information generated. Doesn't include resources size used by a unit.

The project is a single form application (no additional controls used). Lazarus used is a Trunk version of 23 Jan 2020.

No optimization was used during compilation.

Size Lib Package Category File name
238704 lcl base controls.pp
144064 lcl base graphics.pp
111280 lcl ws win32/win32int.pp
106880 lcl base forms.pp
98000 rtl rtl system.pp
94432 lcl base intfgraphics.pas
90352 lcl base comctrls.pp
90224 rtl rtl classes.pp
59552 lcl base stdctrls.pp
51888 lcl base interfacebase.pp
50336 rtl rtl ../win/sysutils.pp
37728 lcl base imglist.pp
35456 rtl rtl rtl-objpas/src/inc/variants.pp
33232 lcl lazutils lazutf8.pas
31456 lcl base extctrls.pp
30912 fcl image fcl-image/src/fpreadtiff.pas
30816 lcl ws win32/win32wscomctrls.pp
30800 lcl ws win32/win32wsmenus.pp
28832 lcl base menus.pp
25984 lcl base maskedit.pp
24480 rtl rtl rtl-objpas/src/win/varutils.pp
24048 lcl base dialogs.pp
23632 fcl json fcl-json/src/fpjson.pp
23456 lcl ws win32/win32wsstdctrls.pp
23408 lcl base lclintf.pas
18448 lcl base lresources.pp
17904 lcl lazutils textstrings.pas
16736 fcl image fcl-image/src/fpcanvas.pp
16544 lcl lazutils graphtype.pp
13408 lcl base themes.pas
13168 lcl ws win32/win32proc.pp
13136 fcl image fcl-image/src/pixtools.pp
13088 lcl lazutils lazloggerbase.pas
12368 lcl lazutils laz_avl_tree.pp
11872 fcl image fcl-image/src/fpimage.pp
11776 lcl base actnlist.pas
11488 lcl base buttons.pp
10576 rtl debug ../inc/heaptrc.pp
10544 lcl ws win32/win32wsbuttons.pp
10464 rtl rtl ../objpas/typinfo.pp
9744 fcl image fcl-image/src/fpreadpng.pp
9696 lcl base graphmath.pp
9536 lcl lazutils lazlogger.pas
9424 lcl lazutils lazfileutils.pas
9296 lcl base buttonpanel.pas
9248 fcl image fcl-image/src/fpwritetiff.pas
8864 lcl base clipbrd.pp
8768 lcl ws win32/win32extra.pas
8768 fcl jpeg pasjpeg/src/jquant2.pas
8720 fcl jpeg pasjpeg/src/jdmarker.pas
8160 fcl image fcl-image/src/fpwritepng.pp
8016 fcl image fcl-image/src/fpwritebmp.pp
7792 fcl zlib paszlib/src/trees.pas
7648 fcl zlib paszlib/src/zdeflate.pas
7536 fcl image fcl-image/src/ellipses.pp
7472 lcl ws win32/win32wsforms.pp
7456 fcl image fcl-image/src/fpreadbmp.pp
7328 lcl ws win32/win32themes.pas
6880 fcl jpeg pasjpeg/src/jdcoefct.pas
6864 lcl base lclproc.pas
6624 lcl lazutils lazutf16.pas
6400 lcl ws win32/win32wscontrols.pp
6240 fcl zlib paszlib/src/infblock.pas
5920 lcl base lclrescache.pas
5792 fcl image fcl-image/src/fpreadjpeg.pas
5728 fcl image fcl-image/src/fppixlcanv.pp
5680 fcl ws win32/win32wsspin.pp
5648 lcl lazutils maps.pp
5632 lcl ws win32/win32wsimglist.pp
5568 fcl jpeg pasjpeg/src/jchuff.pas
5520 fcl jpeg pasjpeg/src/jmemmgr.pas
5392 fcl jpeg pasjpeg/src/jquant1.pas
5264 fcl image fcl-image/src/fpreadgif.pas
5232 fcl jpeg pasjpeg/src/jdphuff.pas
5152 lcl base imagelistcache.pas
5104 lcl base widgetset/wsstdctrls.pp
5008 fcl json fcl-json/src/jsonscanner.pp
4992 lcl base widgetset/wslclclasses.pp
4880 fcl jpeg pasjpeg/src/jcphuff.pas
4848 fcl utils fcl-base/src/contnrs.pp
4816 fcl image fcl-image/src/fptiffcmn.pas
4768 rtl debug ../inc/lnfodwrf.pp
4640 fcl jpeg pasjpeg/src/jcmaster.pas
4592 fcl jpeg pasjpeg/src/jdhuff.pas
4512 fcl jpeg pasjpeg/src/jcparam.pas
4256 fcl jpeg pasjpeg/src/jcsample.pas
4208 lcl base widgetset/wscomctrls.pp
4160 fcl zlib paszlib/src/infcodes.pas
3936 fcl zlib paszlib/src/inftrees.pas
3616 fcl jpeg pasjpeg/src/jcmarker.pas
3584 fcl zlib paszlib/src/zstream.pp
3584 fcl jpeg pasjpeg/src/jidctint.pas
3488 fcl image fcl-image/src/fpwritepnm.pp
3472 fcl image fcl-image/src/fpreadpnm.pp
3360 fcl jpeg pasjpeg/src/jdmerge.pas
3312 lcl base icnstypes.pas
3248 fcl jpeg pasjpeg/src/jccoefct.pas
3216 fcl jpeg pasjpeg/src/jdsample.pas
3168 fcl jpeg pasjpeg/src/jdmainct.pas
3152 lcl base widgetset/wsimglist.pp
3120 lcl base spin.pp
3120 fcl jpeg pasjpeg/src/jdmaster.pas
3104 rtl winapi winunits-base/src/uxtheme.pp
2992 lcl base widgetset/wscontrols.pp
2944 fcl json fcl-json/src/jsonparser.pp
2896 fcl jpeg pasjpeg/src/jccolor.pas
2848 fcl zlib paszlib/src/zinflate.pas
2832 fcl jpeg pasjpeg/src/jdcolor.pas
2816 fcl jpeg pasjpeg/src/jcdctmgr.pas
2752 lcl lazutils lazmethodlist.pas
2736 fcl jpeg pasjpeg/src/jidctred.pas
2704 rtl debug ../inc/exeinfo.pp
2688 fcl jpeg pasjpeg/src/jidctfst.pas
2576 fcl jpeg pasjpeg/src/jidctflt.pas
2496 rtl rtl ../objpas/fgl.pp
2464 fcl jpeg pasjpeg/src/jdinput.pas
2448 lcl ws win32/win32wsfactory.pas
2352 fcl image fcl-image/src/fpwritejpeg.pas
2320 fcl jpeg pasjpeg/src/jcprepct.pas
2208 lcl lazutils lclclasses.pp
2032 fcl jpeg pasjpeg/src/jerror.pas
2016 rtl winapi winunits-base/src/multimon.pp
2016 fcl jpeg pasjpeg/src/jfdctint.pas
1952 rtl winapi windows.pp
1888 fcl jpeg pasjpeg/src/jdapimin.pas
1872 fcl image fcl-image/src/clipping.pp
1824 fcl zlib paszlib/src/inffast.pas
1808 lcl base extendedstrings.pas
1712 lcl base customtimer.pas
1648 lcl base helpintfs.pas
1584 fcl jpeg pasjpeg/src/jfdctfst.pas
1584 fcl jpeg pasjpeg/src/jfdctflt.pas
1584 rtl rtl ../objpas/math.pp
1472 fcl jpeg pasjpeg/src/jdpostct.pas
1360 fcl jpeg pasjpeg/src/jdapistd.pas
1280 fcl utils fcl-base/src/custapp.pp
1200 lcl lazutils lazfilecache.pas
1152 lcl base widgetset/wsmenus.pp
1152 lcl lazutils lazclasses.pas
1120 fcl jpeg pasjpeg/src/jddctmgr.pas
1056 lcl base widgetset/wsforms.pp
1040 lcl lazutils integerlist.pas
1024 fcl jpeg pasjpeg/src/jcapimin.pas
992 lcl base lcltype.pp
848 lcl lazutils lazstringutils.pas
832 fcl image fcl-image/src/fpimgcanv.pp
768 lcl base widgetset/wsproc.pp
688 rtl rtl ../objpas/types.pp
672 lcl ws win32/win32wsdialogs.pp
672 lcl lazutils lazutf8classes.pas
656 fcl jpeg pasjpeg/src/jcmainct.pas
656 rtl rtl ../inc/fpintres.pp
640 lcl lazutils laztracer.pas
624 fcl jpeg pasjpeg/src/jdatasrc.pas
560 fcl image fcl-image/src/fpimgcmn.pp
528 fcl zlib paszlib/src/infutil.pas
528 fcl utils fcl-base/src/syncobjs.pp
512 lcl base lclmessageglue.pas
496 fcl jpeg pasjpeg/src/jutils.pas
496 fcl jpeg pasjpeg/src/jdatadst.pas
480 fcl jpeg pasjpeg/src/jcapistd.pas
464 rtl winapi winunits-base/src/commctrl.pp
432 lcl base widgetset/wsextctrls.pp
432 fcl zlib paszlib/src/zbase.pas
384 fcl jpeg pasjpeg/src/jmemnobs.pas
368 fcl jpeg pasjpeg/src/jcomapi.pas
336 rtl rtl ../objpas/objpas.pp
288 lcl base widgetset/wsdialogs.pp
288 fcl xml fcl-xml/src/htmldefs.pp
256 fcl jpeg pasjpeg/src/jinclude.pas
256 fcl jpeg pasjpeg/src/jcinit.pas
256 lcl lazutils lcltaskdialog.pas
256 lcl lazutils laz2_xmlread.pas
224 lcl base widgetset/wsspin.pp
208 lcl base widgetset/wsbuttons.pp
208 fcl zlib paszlib/src/adler.pas
192 rtl rtl ../objpas/unicodedata.pas
144 lcl lazutils avglvltree.pas
128 lcl base widgetset/wsreferences.pp
112 lcl lazutils stringhashlist.pas
112 main app project1.lpr
96 rtl winapi ../win/windirs.pp
96 rtl winapi ../win/dos.pp
80 lcl base lclversion.pas
80 lcl base lclplatformdef.pas
80 lcl base forms/calcform.pas
80 lcl lazutils fileutil.pas
64 lcl ws win32/interfaces.pp
64 lcl lazutils lazutilities.pas
64 lcl lazutils laz2_dom.pas
64 rtl rtl ../inc/strings.pp
48 lcl lazutils lconvencoding.pas
48 lcl lazutils lazsysutils.pas
48 lcl lazutils laz2_xmlutils.pas
48 lcl lazutils fpcadds.pas
48 fcl utils fcl-base/src/rttiutils.pp
32 fcl utils fcl-base/src/gettext.pp
  • Note, that zlib is used for PNG images reading and writting

by Lib

lib code percentage
rtl 340752 15.46%
lcl 1491184 67.75%
fcl 372176 16.88%
main 122 0.01%
Total 2204234 100%

See Also