phc-discussions - RE: [PHC] Specification of a modular crypt format (2)

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Date: Tue, 6 Oct 2015 04:19:30 +0000
From: Peter Gutmann <pgut001@...auckland.ac.nz>
To: "discussions@...sword-hashing.net" <discussions@...sword-hashing.net>
Subject: RE: [PHC] Specification of a modular crypt format (2)

Alexander Cherepanov <ch3root@...nwall.com> writes:

>Hence a compiler is free to assume that this case is impossible (i.e.
>that "value <= INT_MAX / 10") and to use this knowledge to optimize the
>code in the program after this expression and even before it. 

Fair enough.  I was assuming the compiler would see that valTmp wasn't live 
until after the value range check, so would move the code down to where it 
was used before working with it.  The second link you provided (thanks, very 
interesting, I'd seen other discussions on this but not that one) is kinda 
scary, particularly the Dead Code Elimination vs. Redundant Null Check 
Elimination race condition.

>This has (almost) nothing to do with unsigned numbers. 

Ah, OK, I was assuming it was a side-effect of moving things between 
different types...

>Both gcc 4.9 -O2 and clang 3.5 -O1 produce binaries which print a
>negative number.

Ouch!

>https://gcc.gnu.org/bugzilla/show_bug.cgi?id=30475

Ugh, that matches other threads I've seen involving the gcc developers, 
they'll argue till they're blue in the face that having gcc break code is
valid because if you squint at the spec just right then the behaviour isn't
explicitly disallowed, but won't lift a finger to do the right thing.

Maybe I should just tell everyone to use icc, which doesn't seem to have
these problems (and generally produces better code than gcc to boot).  Or
Visual Studio, for which I still need to check what it does when I get to
a Windows box with it installed.

Peter.