Recent posts

#71

Bug reports / Re: optimiser emits ud2 (inval...

Last post by John Z - September 02, 2026, 09:58:19 AM

Hi KEL26,

Wow what can I say? Thank you!
Exemplary, superb, an excellent piece of work.
Thanks for your work and help and I appreciate the
time you spent doing the extra extensive documentation.

John Z

#72

Beginner questions / Re: Working Directory woes

Last post by MrBcx - September 02, 2026, 01:37:50 AM

Don't delete a good example for others to see to not give up.

That's good advice. 

If you're not making mistakes then you're not learning anything.

... and ...

You gotta break a few eggs, if you wanna make an omelet!

#73

Bug reports / Re: optimiser emits ud2 (inval...

Last post by KEL26 - September 02, 2026, 12:57:34 AM

Code Select Expand


The defect, stated exactly,

At -Os, -Ot and -Ox, Pelles C 14.50 for Windows x64 can replace the
constant divisor K of an integer remainder by K modulo 256.

I attach a zipped folder (edited) showing how to build a bug/defect free zlib (no source patch is required) and folder of code which shows evidence for this bug and my bug tracing code!

Hope it helps others - and hope soon this excellent IDE and compiler become open source - it would make the process of bug tracking and fixing super-fast - I am from a Linux background.



#74

Beginner questions / Re: Working Directory woes

Last post by John Z - September 01, 2026, 09:24:22 PM

Congratulations PhilG57!,

You stuck it out and figured it out!  That is what counts, and is learning.
Don't delete a good example for others to see to not give up.

Cheers,
John Z

#75

Bug reports / Re: optimiser emits ud2 (inval...

Last post by John Z - September 01, 2026, 09:21:51 PM

Hi KEL26,

More than happy to share my code John Z  , I need to know how to do that - anyone guide me or direct me to instructions please?

Thanks very much!

Pelles makes it fairly easy.  Under Projects Menu there is a ZIP File menu selection.  This will ZIP the entire currently open project (zlib) into one file which then can be posted as an attachment, at the bottom of a post.  Up to four attachments can be made on one posting.

If the file is too large delete the object directory from the zip file.

This is also a convenient method to archive and store inactive projects or to keep back-up versions.  Unzip with standard tools and the project continues from where it was.  Also no need to unzip into the same path/structure.

Appreciate the help!

John Z

#76

Bug reports / Re: optimiser emits ud2 (inval...

Last post by KEL26 - September 01, 2026, 09:01:45 PM

Thank you TimoVJL for looking at this again.

I have built your version and I am afraid I cannot reproduce your result. On
Pelles C 14.50.0 it prints 4060, not 19900, and the object code shows why.

Code Select Expand

pocc -Tx64-coff -std:C17 -Ze -Zx -Ot -W1 timo.c
checksum 4060 (correct 19900)

Unoptimised, the same file prints 19900.


WHAT THE OBJECT CODE SHOWS
--------------------------
podump /DISASM on the -Ot object, x64:

Code Select Expand

mov     r8b,2C          ; 0x2C is 44
mov     al,dl
xor     ah,ah
div     r8b             ; 8-bit unsigned divide
mov     al,ah           ; take the remainder
mov     byte ptr [rcx],al

and the same file built for x86:

Code Select Expand

mov     cl,2C           ; 0x2C is 44 here too
mov     al,bl
xor     ah,ah
div     cl
mov     al,ah
mov     byte ptr [esi],al

The source says % 300. The emitted code divides by 44. 300 modulo 256 is 44,
and the divide is the 8-bit form. So the pointer store does not avoid the
rewrite: it is present on both targets and in both spellings.

I also built the array-subscript version with the identical declaration order,
and a pointer version with the array declared first. All three give 4060. So
neither the store form nor the position of the array in the frame changes
anything, and I do not think the stack placement is involved.


WHY I THINK YOUR RUN PRINTED 19900
----------------------------------
Your post mentions that it also gives 19900 with

Code Select Expand

sum += (*data)++;

That line does not read the array. It reads data[0] two hundred times, adding
the old value and incrementing it each time. Starting from data[0] == 0 it
sums 0+1+2+...+199, which is exactly 19900, whatever the other 199 bytes
contain.

I checked this rather than assuming it. Filling the array with 0xFF on
purpose, setting only data[0] to zero, and then running that loop:

Code Select Expand

for (i = 0; i < 200; i++) data[i] = 0xFF;   /* deliberately wrong */
data[0] = 0;
for (i = 0; i < 200; i++) sum += (*data)++;
checksum 19900

19900 from an array that is wrong in every byte. So that particular check
cannot distinguish a good build from a bad one, and I suspect it is where the
19900 came from.

If you ran the *pdata++ version with sum += *pdata++ and still saw 19900,
then something in your settings differs from mine in a way I would very much
like to know about, because it would bound the fault further. Would you say
which target and which optimisation setting you used?


WHERE THIS LEAVES THE WORKAROUND
--------------------------------
For anyone hitting this today, the form I would suggest is still

Code Select Expand

int t = i % 300;
data[i] = (unsigned char)t;

not because it dodges the trigger, but because the remainder is computed in
int and stored in an int object, so the narrowing-to-unsigned-char condition
is never met at all. It is correct at every optimisation level I have tested,
and it stays correct for a reason that does not depend on which shapes this
release happens to recognise.

I would avoid % 256u for the same reason in reverse: it works, but only
because the rewrite does not reach the unsigned-literal path in 14.50, and
that is a property of the release rather than of the language.

Thanks again for minimising the original case. That edit is what moved the
fault to compile time and gave us the #2803 message to work backwards from.
===========================================================================

#77

Bug reports / Re: optimiser emits ud2 (inval...

Last post by TimoVJL - September 01, 2026, 07:42:46 PM

Code Select Expand

int __cdecl printf(const char * restrict format, ...);
int __cdecl main(void)
{
    int i;
    unsigned long sum = 0;
    unsigned char *pdata;    // pointer to data
    unsigned char data[200];    // safer stack place at end ?

    pdata = data;
    for (i = 0; i < 200; i++)
        *pdata++ = (unsigned char)(i % 300);

    pdata = data;
    for (i = 0; i < 200; i++)
        sum += *pdata++;
        //sum += (*data)++;

    printf("checksum %lu (correct 19900)\n", sum);
    return 0;
}
also with

Code Select Expand

sum += (*data)++;output

Code Select Expand

checksum 19900 (correct 19900)
EDIT: a simple check

Code Select Expand

int __cdecl printf(const char * restrict format, ...);
int __cdecl main(void)
{
unsigned char *pdata; // pointer to data
int i;
unsigned char data[200];

pdata = data;
for (i = 0; i < 200; i++)
{
*pdata = (unsigned char)(i % 300);
if (*pdata != i)
{
printf("\nerror: %u != %u\n", i, *pdata);
break;
}
printf("%u ", *pdata);
pdata++;
}
return 0;
}



#78

Bug reports / Re: optimiser emits ud2 (inval...

Last post by KEL26 - September 01, 2026, 05:54:17 PM

Thank you Vortex, but the [ i ] is still there — the forum ate it.   [ i ] is the BBCode italics tag, so outside a code block 'data [ i ] =' ... renders as 'data = ' .... My source is correct. Reposting inside code tags:

Code Select Expand

        data[i] = (unsigned char)(i % 300);

Apologies — I should have wrapped the whole report in code tags. Anywhere else in the post that reads 'data = '  should read ' data [ i ] = '

The rest of the thread: every line with data [ i ] outside a code block will have been mangled the same way.


More than happy to share my code John Z  , I need to know how to do that - anyone guide me or direct me to instructions please?

#79

Bug reports / Re: optimiser emits ud2 (inval...

Last post by John Z - September 01, 2026, 12:48:33 PM

HI KEL26,

First, thank you for Pelles C — it has just carried a full port of CFITSIO
4.7.0 (NASA's FITS library, 59 sources), zlib, libpng and FFTW 3.3.11 to a
clean finish, so the report below comes from a codebase your compiler
otherwise handles admirably.

If you are allowed, and if you don't mind, could you please post the pelles .ppj for the zlib build?
If not, no worries, thought I'd ask because I tried to build this in the past but seemed to run into issues.
 
Thanks either way for posting on the forum!

John Z

#80

Bug reports / Re: optimiser emits ud2 (inval...

Last post by Vortex - September 01, 2026, 11:22:12 AM

There is something unclear about this line :

Code Select Expand

  data = (unsigned char)(i % 300);
Whay not this one?

Code Select Expand

  data[i] = (unsigned char)(i % 300);