Updated getting started guide for visual studio + cmake

[aaron.ballman]

Summary

It took me a while to track this down and I figured I'd save someone else the time. By default, CMake uses the 32-bit toolchain on Windows, even if generating a 64-bit solution. Given the size of LLVM's code base, this can lead to quite a few link errors with the linker running out of memory. If you pass -Thost=x64 when generating the Visual Studio solution, then the resulting project files will use the 64-bit toolchain and the toolchain no longer runs out of memory.

I will add some similar wording to Clang's getting started guide as well.

[zturner]

Can we just error out if this is not specified and have CMake print the error message? CMAKE_GENERATOR_TOOLSET contains the value at generation time, and we can just check the value and print an error. Given that you can't successfully build LLVM / clang / etc without it, we might as well fail fast.

[aaron.ballman]

In D33546#764507, @zturner wrote:

Can we just error out if this is not specified and have CMake print the error message? CMAKE_GENERATOR_TOOLSET contains the value at generation time, and we can just check the value and print an error. Given that you can't successfully build LLVM / clang / etc without it, we might as well fail fast.

We can successfully build LLVM/clang/etc without it (I've done so for *far* too long), but you may get some sporadic "linker out of memory" errors (recompiles will eventually link, however). Also, not everyone may want to use the native toolset. I'm thinking about people who may have something like 8GB of RAM, where the x86 linker is less likely to throttle the machine than the x64 linker.

Since it does work out of the box, why force users to use an extra command line argument?

[zturner]

In D33546#764521, @aaron.ballman wrote:

In D33546#764507, @zturner wrote:

Can we just error out if this is not specified and have CMake print the error message? CMAKE_GENERATOR_TOOLSET contains the value at generation time, and we can just check the value and print an error. Given that you can't successfully build LLVM / clang / etc without it, we might as well fail fast.

We can successfully build LLVM/clang/etc without it (I've done so for *far* too long), but you may get some sporadic "linker out of memory" errors (recompiles will eventually link, however). Also, not everyone may want to use the native toolset. I'm thinking about people who may have something like 8GB of RAM, where the x86 linker is less likely to throttle the machine than the x64 linker.

Since it does work out of the box, why force users to use an extra command line argument?

I've never successfully built clang without using the x64 host toolchain. At the very least, I think we should print a warning.

[aaron.ballman]

In D33546#764526, @zturner wrote:

In D33546#764521, @aaron.ballman wrote:

In D33546#764507, @zturner wrote:

Can we just error out if this is not specified and have CMake print the error message? CMAKE_GENERATOR_TOOLSET contains the value at generation time, and we can just check the value and print an error. Given that you can't successfully build LLVM / clang / etc without it, we might as well fail fast.

We can successfully build LLVM/clang/etc without it (I've done so for *far* too long), but you may get some sporadic "linker out of memory" errors (recompiles will eventually link, however). Also, not everyone may want to use the native toolset. I'm thinking about people who may have something like 8GB of RAM, where the x86 linker is less likely to throttle the machine than the x64 linker.

Since it does work out of the box, why force users to use an extra command line argument?

I've never successfully built clang without using the x64 host toolchain. At the very least, I think we should print a warning.

I've managed it for five years, entirely by accident (and not without frustration, hence the patch), but a warning seems like a reasonable idea.

[aaron.ballman]

Now, with CMake warning. This is only needed for the llvm cmake files (since Clang relies on them anyway).

I couldn't find a way to tell if the host system architecture is not 64-bit in CMake (we could skip the diagnostic on 32-bit hosts), so if someone knows a good incantation for that, it could be useful. I was able to find ways if the target architecture is 64-bit, but that's not helpful since a 32-bit host can still target a 64-bit architecture.

[zturner]

It might be overkill, but CMAKE_HOST_SYSTEM_PROCESSOR probably contains what you need. In any case, lgtm even without doing that.

Presumably you tested this and confirmed that the warning shows up if and only if you don't specify -Thost=x64?

[aaron.ballman]

In D33546#764842, @zturner wrote:

It might be overkill, but CMAKE_HOST_SYSTEM_PROCESSOR probably contains what you need. In any case, lgtm even without doing that.

I found that, but it spits out the friendly name of the processor, which I'd have to parse for the pertinent information. That seemed too fragile for my tastes.

Presumably you tested this and confirmed that the warning shows up if and only if you don't specify -Thost=x64?

Yes. In my tests, if you start from a clean build directory and elide the -T entirely, you get a warning. If you specify -T<anything>, you do not get the warning. Once you specify -Thost=x64, if you reconfigure and leave the -T option off, CMAKE_GENERATOR_TOOLSET still retains the host=x64 and the project files are sane.

One thing to note: -Tv140 will use the MSVC 2015 32-bit toolset and not generate a warning here. However, I suspect that if you are specifying the -T flag, you already know what you want to have happen.

[aaron.ballman]

Commit in r303912

docs/GettingStartedVS.rst

    * CMake generates project files for all build types. To select a specific
      build type, use the Configuration manager from the VS IDE or the 
      ``/property:Configuration`` command line option when using MSBuild.
+   * By default, the Visual Studio project files generated by CMake use the
+     32-bit toolset. If you are developing on a 64-bit version of Windows and
+     want to use the 64-bit toolset, pass the ``-Thost=x64`` flag when
+     generating the Visual Studio solution. This requires CMake 3.8.0 or later.
 
 6. Start Visual Studio