Date: Sun, 13 Nov 2022 10:42:41 -0500 From: Paul Procacci <pprocacci@gmail.com> To: Daniel Cervus <DanieltheDeer@outlook.com> Cc: "freebsd-questions@freebsd.org" <freebsd-questions@freebsd.org> Subject: Re: Question about AMD64 ABI Message-ID: <CAFbbPuge7D4Cky5ucrBg1%2Bp8QLj7t2RMAY-440d935%2BYvyddFw@mail.gmail.com> In-Reply-To: <TYWP286MB2667168BD04FDA3B30DF0435B8029@TYWP286MB2667.JPNP286.PROD.OUTLOOK.COM> References: <CAFbbPujfaSZ%2BxGsKPL4J-arydLCr7=YGyrBTt18Cg8q16z3Tdg@mail.gmail.com> <TYWP286MB2667FDC5425C52AD3A492052B8029@TYWP286MB2667.JPNP286.PROD.OUTLOOK.COM> <CAFbbPuiQHDa6GdmD3BZDfgXJ9_pmQ1g2FYsQ=gvxK12veq%2BSfw@mail.gmail.com> <TYWP286MB2667168BD04FDA3B30DF0435B8029@TYWP286MB2667.JPNP286.PROD.OUTLOOK.COM>
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On Sun, Nov 13, 2022 at 7:41 AM Daniel Cervus <DanieltheDeer@outlook.com>
wrote:
> Sorry if I didn=E2=80=99t make myself clear. in other words, I mean if =
a
> function prototype requires a parameter less than 64 bits (such as int,
> short...), since the ABI requires it to be passed by a 64-bit register,
> will the function completely ignore the unused bits? Can I safely leave t=
he
> garbage in upper bits? Or I have to expand everything to 64 bits?
>
>> 64 bits (such as int, short...), since the ABI requires it to be passed
by a 64-bit register, will the function completely ignore the unused bits?
Can I safely leave the garbage in upper bits? Or I have to expand
everything to 64 bits?
This depends on the function and who wrote it.
What I mean specifically is, if you're a fledgling asm programmer and you
wrote a function with the intention of addressing a 32-bit register, but
instead you address it as a 64-bit register, then that's a problem because
whatever results you get are erroneous.
If instead you're leaning on a compiler to build the function body, then
the compiler will undoubtedly do the right thing.
The above is a long winded way for me to say: For most purposes and in my
experience has always been that it's safe to ignore higher bits of passed
32-bit registers.
Thanks,
Paul
--=20
__________________
:(){ :|:& };:
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<div dir=3D"ltr"><div dir=3D"ltr"><div dir=3D"ltr"><br></div><br><div class=
=3D"gmail_quote"><div dir=3D"ltr" class=3D"gmail_attr">On Sun, Nov 13, 2022=
at 7:41 AM Daniel Cervus <<a href=3D"mailto:DanieltheDeer@outlook.com">=
DanieltheDeer@outlook.com</a>> wrote:<br></div><blockquote class=3D"gmai=
l_quote" style=3D"margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,20=
4,204);padding-left:1ex">=C2=A0 Sorry if I didn=E2=80=99t make myself clear=
. in other words, I mean if a function prototype requires a parameter less =
than 64 bits (such as int, short...), since the ABI requires it to be passe=
d by a 64-bit register, will the function completely ignore the unused bits=
? Can I safely leave the garbage in upper bits? Or I have to expand everyth=
ing to 64 bits?<br>
</blockquote></div><br clear=3D"all">>> 64 bits (such as int, short..=
.), since the ABI requires it to be passed=20
by a 64-bit register, will the function completely ignore the unused=20
bits? Can I safely leave the garbage <span class=3D"gmail-Lm gmail-ng">in u=
pper</span> bits? Or I have to expand=20
everything to 64 bits?<br><br></div><div>This depends on the function and w=
ho wrote it. <br><br></div><div>What I mean specifically is, if you're =
a fledgling asm programmer and you wrote a function with the intention of a=
ddressing a 32-bit register, but instead you address it as a 64-bit registe=
r, then that's a problem because whatever results you get are erroneous=
.<br><br></div><div>If instead you're leaning on a compiler to build th=
e function body, then the compiler will undoubtedly do the right thing.<br>=
<br></div><div>The above is a long winded way for me to say:=C2=A0 For most=
purposes and in my experience has always been that it's safe to ignore=
higher bits of passed 32-bit registers.<br></div><div><br></div><div>Thank=
s,</div><div>Paul<br><br></div><div dir=3D"ltr">-- <br><div dir=3D"ltr" cla=
ss=3D"gmail_signature">__________________<br><br>:(){ :|:& };:</div></d=
iv></div>
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