Comments on: Efficient C Tips #6 – Don’t use the ternary operator

By: Nigel Jones

Nigel Jones — Wed, 18 Jan 2012 00:26:40 +0000

Yes. I have come across plenty of compilers that perform as gcc has done here. However I have also come across a number of compilers, particularly for low end embedded systems where this is *not* the case.

By: Jörg Seebohn

Jörg Seebohn — Tue, 17 Jan 2012 13:49:26 +0000

I’ve disassembled the generated object module from the following C-module constisting only of these
two functions. The compiler is gcc 4.5.2 on a x86 processor.

int f1(int a, int b, int c, int d)
{
int y ;
y = (a > b) ? c : d ;
return y ;
}

int f2(int a, int b, int c, int d)
{
int y ;
if (a > b) { y = c ; } else { y = d ; }
return y ;
}

The generated assembler code is exactly the same for both functions.
Even better cause of the CMOVcc instruction of the x86 processor the
generated code is branchless.

Disassembly of section .text:

00000000 :
0: 55 push %ebp
1: 89 e5 mov %esp,%ebp
3: 8b 45 0c mov 0xc(%ebp),%eax
6: 39 45 08 cmp %eax,0×8(%ebp)
9: 8b 45 14 mov 0×14(%ebp),%eax
c: 0f 4f 45 10 cmovg 0×10(%ebp),%eax
10: 5d pop %ebp
11: c3 ret

00000012 :
12: 55 push %ebp
13: 89 e5 mov %esp,%ebp
15: 8b 45 0c mov 0xc(%ebp),%eax
18: 39 45 08 cmp %eax,0×8(%ebp)
1b: 8b 45 14 mov 0×14(%ebp),%eax
1e: 0f 4f 45 10 cmovg 0×10(%ebp),%eax
22: 5d pop %ebp
23: c3 ret

By: Jill

Jill — Tue, 17 Jan 2012 03:19:35 +0000

“I find this frustrating, as I’ve consumed 8 lines doing what is more easily and elegantly performed in 1 line.”

You mean you’ve expressed more clearly in 4 lines what you could have obfuscated in 1 line.

By: Nigel Jones

Nigel Jones — Fri, 03 Dec 2010 13:24:35 +0000

I will see if I can dig up some examples.

By: Peter

Peter — Fri, 03 Dec 2010 07:10:25 +0000

Hello Nigel,
Interesting post. Can you add some examples of code generated for the ternary operator and for the if/else construct, for comparison?

That would make the post more complete.
Thanks!

By: Doug

Doug — Wed, 28 Jul 2010 00:20:20 +0000

My personal favorite “do not use unless absolutely necessary” construct is modulo. Modulo looks simple and elegant when written, but try it out — write a simple modulo construct, compile it and look at the assembly output. It’s horrific. Think the problem through and simplify it instead. I haven’t yet found a modulo solution I couldn’t implement much more simply and completely by rethinking the problem, resulting in code that’s as accurate, uses less memory, and runs quicker.

By: Greg Nelson

Greg Nelson — Wed, 24 Feb 2010 19:47:03 +0000

The printf() comment reminds me of something else… Nigel, since you said you are often interested in people's weird coding conventions, this one might fit the bill. Want to guess why I write:char tmp1[10];char tmp2[10];char tmp3[10];char final[50];snprintf(tmp1, "%7.1f", f1);snprintf(tmp2, "%7.1f", f2);snprintf(tmp3, "%7.1f", f3);snprintf(final, "%s %s %s", tmp1, tmp2, tmp3);Answer:Because one of the compilers/libraries (still haven't found the culprit) used for our code can't handle multiple double arguments to a varargs call!If I do it the conventional way with "%7.1f %7.1f %7.1f" instead, I either get garbage output, or the program hangs.My guess is this has something to do with poor floating point emulation and/or different ways of stacking doubles versus ints/ptrs/etc.

By: bandit

bandit — Tue, 26 Jan 2010 01:50:50 +0000

I use them usually only for printf():printf( "%s", (bletch ? "foo" : "bar") );otherwise I use if() else for clarity. I normally dofoo = 0;if( bar ) foo = 1;because this handles the default case && the exceptional case.

By: ashleigh

ashleigh — Tue, 26 Jan 2010 01:16:26 +0000

I've seen the same as Nigel – often the ternary operator is worse.I still use it where it improves readability of the source and I don't care about performance.I guess the lesson is – think about what you write. Write with maintenance, readability AND performance in mind. Know what the compiler does (ie go in with your eyes open), and then write whats best for the situation you are handling.

By: Mike

Mike — Wed, 20 Jan 2010 16:34:16 +0000

Just ran a test with HC08 and CodeWarrior — ternary operator uses 4 more bytes than if/else in one particular test case.