msvc: pass -Thost=x64 when compiling x64 target

[jordanrh1]

If you try to build rust for --host x86_64-pc-windows-msvc, you will receive the following error when it tries to compile llvm:

Visual Studio generators use the x86 host compiler by
default, even for 64-bit targets. This can result in linker
instability and out of memory errors. To use the 64-bit
host compiler, pass -Thost=x64 on the CMake command line.

This change passes -Thost=x64 to cmake when compiling for x64, which eliminates the error.

[alexcrichton]

Thanks!

[alexcrichton]

Er actually, shouldn't this match on the host target rather than the target target?

[jordanrh1]

Pretty sure 'target' in this case refers to the host target. When I cross compile to thumbv7a-pc-windows-msvc, 'target' is x86_64-pc-windows-msvc.

[alexcrichton]

Hm are you sure? That's what the host variable is for (the host, whereas target should be thumbv7a)

[jordanrh1]

I'm sure. target gets passed to visual_studio_generator(), which uses target to determine which visual studio generator to use. The generated visual studio project targets the host architecture.

[alexcrichton]

Ah ok I think we're talking about different things, I assumed that we'd pass this argument for all compiles (even if you're cross-compiling) to use the 64-bit host compiler by default. But it sounds like what you're targeting this patch for is to only use the 64-bit host compiler when targeting a 64-bit platform, which is also fine.