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From: Michael S <already5chosen@yahoo.com>
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Subject: Re: is Vax addressing sane today
Date: Sun, 29 Sep 2024 13:13:26 +0300
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On Wed, 25 Sep 2024 13:56:40 -0400
EricP <ThatWouldBeTelling@thevillage.com> wrote:

> Terje Mathisen wrote:
> > Kent Dickey wrote:  
> >>
> >> Look at:
> >> https://godbolt.org/z/oMhW55YsK
> >>
> >> Which is this code:
> >>
> >> int add2(int num, int other) {
> >>      return num + other;
> >> }
> >>
> >> Compiled with these options:  -O2 -ftrapv
> >> (-ftrapv is the GCC argument for detect signed overflows and
> >> crash).
> >>
> >> For x86-64 clang 19.1.0:
> >>
> >> add2:
> >>          add     edi, esi
> >>          jo      .LBB0_1
> >>          mov     eax, edi
> >>          ret
> >> .LBB0_1:
> >>          ud1     eax, dword ptr [eax]
> >>
> >> This looks OK: it does a normal add, then branches-on-overflow to
> >> an undefined instruction.
> >>
> >> But x86 has an instruction to trap on overflow directly: INTO.
> >> It's one byte.
> >> And it doesn't use it.
> >>
> >> GCC x86-64 14.2 is even worse:
> >>
> >> add2:
> >>          sub     rsp, 8
> >>          call    __addvsi3
> >>          add     rsp, 8
> >>          ret
> >>
> >> It calls a routine to do all additions which might overflow, and
> >> that routine calls assert() if an overflow occurs.
> >>
> >> The CPU has a trap-on-overflow instruction exactly for this case
> >> (to crash
> >> on detecting an overflow), and compilers don't even use it.
> >>
> >> So even on architectures which have a trap-on-overflow instruction,
> >> compilers don't use it.  
> > 
> > You can only compile in INTO opcodes if you can guarantee that the
> > INT 4 (INTO) trap vector will always be set to a proper handler,
> > and since that isn't part of the ABI, compilers can't depend on it?
> > 
> > I do agree that it would be nice if it did work, barring that clang
> > is doing the best possible alternative, at close to zero cost
> > except for the useless branch predictor table entry wastage.
> > 
> > Terje  
> 
> On x64 in 64-bit mode INTO is among 21 opcodes reassigned as invalid.
> One must use JO to detect signed overflow.
> Others were repurposed, 1-byte INC and DEC 40..4F became the REX
> prefix.
> 
> 

Single-byte form of INTO reassigned. Dual-byte form (CD 04) is here.