Re: [PATCH 1/5] add SWAP macro

[Johannes Schindelin <Johannes.Schindelin@gmx.de>]

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Hi René,

On Mon, 30 Jan 2017, René Scharfe wrote:

> Am 30.01.2017 um 16:39 schrieb Johannes Schindelin:
>
> > On Sat, 28 Jan 2017, René Scharfe wrote:
> >
> > > Add a macro for exchanging the values of variables.  It allows users
> > > to avoid repetition and takes care of the temporary variable for
> > > them.  It also makes sure that the storage sizes of its two
> > > parameters are the same.  Its memcpy(1) calls are optimized away by
> > > current compilers.
> >
> > How certain are we that "current compilers" optimize that away? And
> > about which "current compilers" are we talking? GCC? GCC 6? Clang?
> > Clang 3?  Clang 3.8.*? Visual C 2005+?
> 
> GCC 4.4.7 and clang 3.2 on x86-64 compile the macro to the same object
> code as a manual swap ; see https://godbolt.org/g/F4b9M9.  Both were
> released 2012.

Good. Thank you.

> That website doesn't offer Visual C(++), but since you added the
> original macro in e5a94313c0 ("Teach git-apply about '-R'", 2006) I
> guess we have that part covered. ;)

Back then, I was not able to build Git using Visual C *at all*. It
required a Herculean effort to make that happen, and it did not happen
before the Git for Windows project was started in 2007.

So I tried to verify that Visual C optimizes this well, and oh my deity,
this was not easy. In Debug mode, it does not optimize, i.e. the memcpy()
will be called, even for simple 32-bit integers. In Release mode, Visual
Studio's defaults turn on "whole-program optimization" which means that
the only swapping that is going on in the end is that the meaning of two
registers is swapped, i.e. the SWAP() macro is expanded to... no assembler
code at all.

After trying this and that and something else, I finally ended up with the
file-scope optimized SWAP() resulting in this assembly code:

00007FF791311000  mov         eax,dword ptr [rcx]  
00007FF791311002  mov         r8d,dword ptr [rdx]  
00007FF791311005  mov         dword ptr [rcx],r8d  
00007FF791311008  mov         dword ptr [rdx],eax  

However, recent events (including some discussions on this mailing list
which had unfortunate consequences) made me trust my instincts more. And
my instincts tell me that it would be unwise to replace code that swaps
primitive types in the straight-forward way by code that relies on
advanced compiler optimization to generate efficient code, otherwise
producing very suboptimal code.

The commit you quoted embarrasses me, and I have no excuse for it. I would
love to see that myswap() ugliness fixed by replacing it with a construct
that is simpler, and generates good code even without any smart compiler.

Ciao,
Dscho