Compiler fail
lazarus / 13-General

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anna · 2012-08-31 08:41

Compiler converts same slices of asm-code by different ways. See this
message:
http://www.lazarus.freepascal.org/index.php/topic,17968.msg101124.html
#msg101124

As you can see binary data of executable differs (OllyDBG debugger was
used).

Why it was happened?

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theo · 2012-08-31 10:04

I would ask in fpc-devel 
http://freepascal.org/maillist.var

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ludob · 2012-08-31 10:11

The difference is that cheaten uses delphi mode and your testprogram
probably objfpc mode. Delphi mode doesn't generate stackframes anymore
for pure assembler functions.
The missingCode: [Select]PUSH EBP 
MOV EBP,ESP
...
LEAVE is the stackframe code.

See http://wiki.freepascal.org/User_Changes_2.2.0#Pure_assembler_routi
nes_in_Delphi_mode.

Edit: I tried adding {$W+} to force stackframes but that doesn't work
for pure assembler functions.
Edit: but adding {$MODE objfpc} at the top of BigInt18 does work

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anna · 2012-08-31 12:28

Quote from: ludob on August 31, 2012, 12:11:32 pmThe difference is
that cheaten uses delphi mode and your testprogram probably objfpc
mode. Delphi mode doesn't generate stackframes anymore for pure
assembler functions.
The missingCode: [Select]PUSH EBP 
MOV EBP,ESP
...
LEAVE is the stackframe code.

See http://wiki.freepascal.org/User_Changes_2.2.0#Pure_assembler_routi
nes_in_Delphi_mode.

Edit: I tried adding {$W+} to force stackframes but that doesn't work
for pure assembler functions.
Edit: but adding {$MODE objfpc} at the top of BigInt18 does work

Thank you very much . But for what purpose developers made different
processing asm-code in delfi and objfpc?

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ludob · 2012-08-31 13:21

Delphi added a compiler optimisation at one stage breaking iirc TP
behavior. Since Delphi mode is supposed to be compatible with Delphi,
fpc had to implement 2 different behaviors.

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taazz · 2012-08-31 13:28

Quote from: ludob on August 31, 2012, 03:21:54 pmDelphi added a
compiler optimisation at one stage breaking iirc TP behavior. Since
Delphi mode is supposed to be compatible with Delphi, fpc had to
implement 2 different behaviors.

err, Delphi compatibility in what level? Is it really needed an asm
compatibility and for what reason? I am under the impression that
compatability is on the source code level aka the parser and I don't
understand why should it have asm compatibility as well.

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ludob · 2012-08-31 13:46

QuoteIs it really needed an asm compatibility and for what reason?If
you want to run delphi code that uses assembler functions the compiler
would better generate the same function prologue.  When a stackframe
is used, pushed parameters and local variables on win32 are relative
to EBP. When no stackframe is used, they are relative to ESP.

This is just the same as any other calling convention fpc is trying to
be compatible with. The compiler has to set up all registers as
defined in the convention. Nothing says that this has to be done in
this case with Code: [Select]PUSH EBP 
MOV EBP,ESP
...
LEAVEbut why re-invent the wheel when these instructions are used
everywhere to obtain the expected result?

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anna · 2012-09-01 08:17

Quote from: ludob on August 31, 2012, 03:46:24 pmQuoteIs it really
needed an asm compatibility and for what reason?If you want to run
delphi code that uses assembler functions the compiler would better
generate the same function prologue.  When a stackframe is used,
pushed parameters and local variables on win32 are relative to EBP.
When no stackframe is used, they are relative to ESP.

Stack is always pointed by ESP.

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ludob · 2012-09-01 08:40

QuoteStack is always pointed by ESP.Yes, but when a stackframe is used
the procedure will reference pushed parameters and local variables
relative to ebp instead of esp. That is why your assembler procedures
crashed without a stackframe. They referenced a EBP that is not set up
correctly.
For more info see fe. http://en.wikibooks.org/wiki/X86_Disassembly/Fun
ctions_and_Stack_Frames or http://www.guidogybels.eu/asmch3.html