How do I find out why g++ takes a very long time o

Question

I am building a lot of auto-generated code, including one particularly large file (~15K lines), using a mingw32 cross compiler on linux. Most files are extremely quick, but this one large file takes an unexpectedly long time (~15 minutes) to compile.

I have tried manipulating various optimization flags to see if they had any effect, without any luck. What I really need is some way of determining what g++ is doing that is taking so long. Are there any (relatively simple) ways to have g++ generate output about different phases of compilation, to help me narrow down what the hang-up might be?

Sadly, I do not have the ability to rebuild this cross-compiler, so adding debugging information to the compiler and stepping through it is not a possibility.

What's in the file:

• a bunch of includes
• a bunch of string comparisons
• a bunch of if-then checks and constructor invocations

The file is a factory for producing a ton of different specific subclasses of a certain parent class. Most of the includes, however, are nothing terribly fancy.

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The results of -ftime-report, as suggested by Neil Butterworth, indicate that the "life analysis" phase is taking 921 seconds, which takes up most of the 15 minutes.

It appears that this takes place during data flow analysis. The file itself is a bunch of conditional string comparisons, constructing an object by class name provided as a string.

We think changing this to point into a map of names to function pointers might improve things a bit, so we're going to try that.

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Indeed, generating a bunch of factory functions (per object) and creating a map from the string name of the object to a pointer to its factory function reduced compile time from the original 15 minutes to about 25 seconds, which will save everyone tons of time on their builds.

Thanks again to Neil Butterworth for the tip about -ftime-report.

Answer 1

What slows g++ down in general are templates. For example Boost loves to use them. This means nice code, great performances but poor compiling speed.

On the other hand, 15min seems extremely long. After a quick googling, it seems that it is a common problem with mingw

Answer 2

One thing to watch during the compile is how much memory your computer has free. If the compiler allocates so much memory that the computer starts swapping, compile time will go way, way up.

If you see that happen, an easily solution is to install more RAM... or just split the file into multiple parts that can be compiled separately.