Tuesday, April 11, 2017

A nifty site to see what a compiler does to C++ code

While investigating a very intermittent unit test failure this week, I noticed an anomaly in our code.  This test is written in c++ and had an extra semicolon in it:

We had a test that did something like this:
Worksheet wb = wbc->GetSheet();
 CPPUNIT_ASSERT(blah blah);

Notice that extra ; in the middle?  Since the test intermittently fails, I was looking for anything unexpected in the code.  This is unexpected, but I also needed to know if it was important.

Matt Godbolt created a terrific site that lets you put in C++ code and see what output various compilers produce.  The site is here  https://gcc.godbolt.org/

You can choose different compilers and I just took a look at gcc 6.3 to see if it would ignore an extra ;. 

Here's my test code:
void test()
int x=1;

And here is the output:
        push    rbp
        mov     rbp, rsp
        mov     DWORD PTR [rbp-4], 1
        pop     rbp

I get the same output with or without the extra semicolon. This is great since I would expect the compiler to get rid of blank commands like this.  Since the compiler does indeed ignore this typo in the code, I can move on to other avenues of investigation.

Give this site a whirl.  You can choose several different compilers and chip options, pass parameters in and so on.  Thanks Matt!

Questions, comments, concerns and criticisms always welcome,

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