Subj : Inline Assembler bug with structs of certain size
To   : borland.public.cpp.borlandcpp
From : Jeremy
Date : Thu Dec 02 2004 03:51 pm

I found an interesting bug in the BC5.0B inline assembler today (after a
couple of days tracking it down). I mention it here because it may still
exist in newer versions.

I have a large struct holding state information for a number of output
objects. The other day I added a new member to the struct & all of a sudden
my code (for an embedded 80188 CPU in a fire alarm panel) started
mysteriously crashing soon after power-up. I eventually traced the problem
to the fact that a couple of code fragments using inline assembler weren't
working any more (were trashing system RAM instead of clearing elements in
the struct).

It turns out that my old structure (prior to the new element) was 19928
bytes long & the new one is 19932. I found that once you get over 19930 (it
works OK again if you're above 20253!) the inline assembly statements using
the structure address simply don't generate code! The following code
illustrates the problem nicely:

--------------------------------------------
#include <stdio.h>

static struct {
 char Data[19930]; // OK
// char Data[19931]; // Bad
// char Data[20253]; // Bad
// char Data[20254]; // OK
} TestStruct1;

static struct {
// char Data[19930]; // OK
 char Data[19931]; // Bad
// char Data[20253]; // Bad
// char Data[20254]; // OK
} TestStruct2;

unsigned asm_test1(void) {
 asm {
  mov  ax,OFFSET TestStruct1
 }
 return _AX;
}

unsigned asm_test2(void) {
 asm {
  mov  ax,OFFSET TestStruct2
 }
 return _AX;
}

void main(void) {

 printf("length of asm_test1 = %u\n", (unsigned)asm_test2 -
(unsigned)asm_test1);
 _AX = 0xEEEE;
 printf("&TestStruct1 = %04x, via asm = %04x\n", &TestStruct1, asm_test1());

 printf("length of asm_test2 = %u\n", (unsigned)main - (unsigned)asm_test2);
 _AX = 0xEEEE;
 printf("&TestStruct2 = %04x, via asm = %04x\n", &TestStruct2, asm_test2());
}
--------------------------------------------

This code generates the following result:

length of asm_test1 = 4
&TestStruct1 = 0882, via asm = 0882
length of asm_test2 = 1
&TestStruct2 = 565c, via asm = eeee

The generated code for the asm_test2() function ends up being a simple RET
instruction! It seems to be pretty much independent of memory model &
optimizations.

I need the code to be in assembler for speed, & preferably near code (I use
large memory model in my app), as the code is called frequently using a
number of module-local variables. Any ideas for a workaround?

Thanks in advance,

Jeremy

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