[LLVMdev] BuildMode

[Reid Spencer]

Hi David,

On Mon, 2007-06-25 at 17:43 -0500, David A. Greene wrote:
> I'm continuing my quest to integrate _GLIBCXX_DEBUG into the config system.
> I now have things at a point where I can configure llvm to build with 
> -D_GLIBCXX_DEBUG and the llvm-gcc will pick up the correct CPPFLAGS
> automatically.

Okay.

> 
> One thing I noticed in the llvm Makefile.rules is this:
> 
> # If DISABLE_ASSERTIONS=1 is specified (make command line or configured),
> # then disable assertions by defining the appropriate preprocessor symbols.
> ifdef DISABLE_ASSERTIONS
>   BuildMode := $(BuildMode)-Asserts
> [...]
> else
> [...]
> endif
> 
> This seems backward to make.  If I configure with --disable-assertions I would
> expect the build to be in a directory called Debug.  If I --enable-asserts
> (the default), I would expect it in Debug-Asserts.  But this logic does just
> the opposite.  So I changed it in my copy to do what I expected (and changed
> the llvm-gcc configuration correspondingly).
> 
> Is this what was intended and the above code is a bug or did I misinterpret
> something?

I think you mis-interpreted something. LLVM has several build modes and
while they are confusing, it makes sense if you look at the details.
Basically, there are three things that are being controlled here:
Optimization (e.g. -O2), Debug code (-D_NDEBUG) and assertions. We have
pre-defined build modes as follows:

Release = optimized + assertions
Debug = assertions + debug (not optimized)
Release-Asserts = optimized (only) 
Debug-Asserts = debug (not optimized, no asserts)

If you build with -disable-assertions you are telling whatever build
mode (Debug or Release) to not include assertions. Consequently you get
either Release-Asserts (Release minus asserts) or Debug-Asserts (Debug
minus Asserts).

If you build with -enable-assertions you restating the default so you
don't get the "minus asserts" suffix.

I would appreciate it if you would not commit the changes you made or
you'll confuse a lot of us. Its already confusing enough.

> 
> Now in the llvm Makefile:
> 
> # NOTE: This needs to remain as the last target definition in this file so
> # that it gets executed last.
> all:: 
> 	$(Echo) '*****' Completed $(BuildMode)$(AssertMode) Build
> ifeq ($(BuildMode),Debug)
> 	$(Echo) '*****' Note: Debug build can be 10 times slower than an
> 	$(Echo) '*****' optimized build. Use 'make ENABLE_OPTIMIZED=1' to
> 	$(Echo) '*****' make an optimized build.
> endif
> 
> This is the only place I can find AssertMode mentioned.  I would much rather
> keep things specified in BuildMode.  I add "-ExpensiveChecks" to BuildMode
> when --enable-expensive-checks is passed to configure (it is off by default).
> This is what turns on -D_GLIBCXX_DEBUG.  That way I can keep around Debug
> builds with and without expensive run-time checks.

Yeah, this appears to be bug in this rule. It shouldn't be using
$(AssertMode) there, just $(BuildMode). Furthermore, the ifeq directive
needs to find "Debug" anywhere in $(BuildMode) not just if its equal to
"Build". That is, you want the warnings to occur when BuildMOde is
"Debug" or "Debug-Asserts" or "Debug+ExpensiveChecks" or any other
variant that includes "Debug".

As a side note, you are ADDING something to the build rather than
subtracting so please use + instead of - in your BuildMode for
--enable-expensive-checks. That is, when you add
--enable-expensive-checks, the BuildMode should be "Debug
+ExpensiveChecks" not "Debug-ExpensiveChecks".


> 
> In summary, it looks like the build system is a little confused and perhaps
> some changes were started in the past but never completed (AssertMode,
> etc.).  I tried to clean things up to be consistent and straightforward.  Does
> it look ok to everyone?

No. I think you're missing the point about - vs + as described above.
The way we define Build mode is as either "Release" or "Debug" followed
by things either added to or subtracted from the default configuration
for those. For example, you could have:

configure --enable-optimized --disable-assertions
--enable-expensive-checks

I would expect the build mode for this to be:

Release-Asserts+ExpensiveChecks

Reid.
> 
>                                             -Dave
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