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Subject: Re: BUG: djgpp should allow 16-byte alignment
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On Friday, 10 March 2017 00:13:25 UTC+1, Louis Santillan (lpsantil AT gmail DOT com) [via djgpp AT delorie DOT com]  wrote:
> Are you unable to use `--mstackrealign`, `-mpreferred-stack-boundary=num`, `-mincoming-stack-boundary=num` [0]?  Looks like you'll also need to force those options if you compile with `-Os` as well.
> 
> 
> 
> 
> [0] https://gcc.gnu.org/onlinedocs/gcc-4.8.5/gcc/i386-and-x86-64-Options.html#i386-and-x86-64-Options  
> 
> 
> On Thu, Mar 9, 2017 at 8:14 AM, jwjag DOT  DOT  DOT  AT gmail DOT com [via dj DOT  DOT  DOT  AT delorie DOT com] <dj DOT  DOT  DOT  AT delorie DOT com> wrote:
> Currently it seems that djgpp does not support 16-byte alignment. Since djgpp-compiled programs can potentially use SSE instructions (either explicitly or implicitly, when compiled with -march=pentium3), I do think this should be supported.
> 
> 
> 
> previous discussion:
> 
> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79935

I was already using -mpreferred-stack-boundary=4 (which implies -mincoming-stack-boundary=4) but that didn't seem to have the intended effect. Most notably in static constructors, the stack alignment is always 8 bytes off. From the disassembly I can tell gcc does its best to keep the stack aligned, so that part works correctly, at least.

Now yesterday I had a problem in a non-static function, where gcc inserted an SSE instruction with a memory operand [ebp-0x58], with (ebp % 0x10 == 0x0c). That doesn't make any sense at all! It's guaranteed to cause a GP fault and gcc should know this.