Channels ▼

The Most Underused Compiler Switches in Visual C++

, March 11, 2014 Microsoft's Visual C++ team explains the nine most underused switches and why you should make sure to include them in your build.
  • E-mail
  • Print

/Qvec-report

With Visual Studio 2012, we also introduced automatic vectorization feature in the compiler. The "auto-vectorizer" tries to make loops in your code run faster by automatically vectorizing your code — that's to say, using the SSE instructions available in all current mainline Intel and AMD chips. Auto-vectorization is enabled by default. You don't need to request this speedup. You don't need to throw a compiler switch. You don't need to set environment variables or registry entries. You don't need to change your C++ code. You don't need to insert #pragmas. The compiler just goes ahead and does it. It all comes for free.

So what does /Qvec-report bring to the table? In order to always maintain functional correctness, the compiler is sometimes conservative and fails automatic vectorization for a loop. The /Qvec-report provides diagnostic information about auto-vectorization, and is equipped with the following reporting levels:

/Qvec-report:1, Outputs an informational message for loops that are vectorized.

/Qvec-report:2, Outputs an informational message for loops that are vectorized and for loops that are not vectorized, together with a reason code.

Additional comments:

If you use C++, it is likely that you care a lot about runtime performance. Recompiling with VS2012 and above brings you that added performance with automatic vectorization. To fine-tune those hot-spots in your application, try out /Qvec-report. Sometimes, you can tweak the way you wrote a loop, allowing the compiler to then auto-vectorize it for you. For more information, see Jim Hogg's series of blogs on this topic.






Currently we allow the following HTML tags in comments:

Single tags

These tags can be used alone and don't need an ending tag.

<br> Defines a single line break

<hr> Defines a horizontal line

Matching tags

These require an ending tag - e.g. <i>italic text</i>

<a> Defines an anchor

<b> Defines bold text

<big> Defines big text

<blockquote> Defines a long quotation

<caption> Defines a table caption

<cite> Defines a citation

<code> Defines computer code text

<em> Defines emphasized text

<fieldset> Defines a border around elements in a form

<h1> This is heading 1

<h2> This is heading 2

<h3> This is heading 3

<h4> This is heading 4

<h5> This is heading 5

<h6> This is heading 6

<i> Defines italic text

<p> Defines a paragraph

<pre> Defines preformatted text

<q> Defines a short quotation

<samp> Defines sample computer code text

<small> Defines small text

<span> Defines a section in a document

<s> Defines strikethrough text

<strike> Defines strikethrough text

<strong> Defines strong text

<sub> Defines subscripted text

<sup> Defines superscripted text

<u> Defines underlined text

Dr. Dobb's encourages readers to engage in spirited, healthy debate, including taking us to task. However, Dr. Dobb's moderates all comments posted to our site, and reserves the right to modify or remove any content that it determines to be derogatory, offensive, inflammatory, vulgar, irrelevant/off-topic, racist or obvious marketing or spam. Dr. Dobb's further reserves the right to disable the profile of any commenter participating in said activities.

 
Disqus Tips To upload an avatar photo, first complete your Disqus profile. | View the list of supported HTML tags you can use to style comments. | Please read our commenting policy.