phosek added a comment.
In D89013#2660407 <https://reviews.llvm.org/D89013#2660407>, @MaskRay wrote:
> Regarding
>
> /usr/include/x86_64-unknown-linux-gnu/c++/v1
> /usr/include/c++/v1
>
> With GCC multiarch, some `include` search paths are preceded by
> machine-os-env specific suffix directories.
> Note: 'vendor' is not there. So you may see `.../include/x86_64-linux-gnu`
> but there is no `.../include/x86_64-{pc,unknown}-linux-gnu`
>
> /usr/local/include/x86_64-linux-gnu # affected by sysroot,
> multiarch, usually nonexistent
> /usr/local/include # affected by sysroot
> /tmp/opt/gcc-debug/include
> /tmp/opt/gcc-debug/lib/gcc/x86_64-pc-linux-gnu/11.0.1/include-fixed
>
> /tmp/opt/gcc-debug/lib/gcc/x86_64-pc-linux-gnu/11.0.1/../../../../x86_64-pc-linux-gnu/include
> /usr/include/x86_64-linux-gnu # affected by sysroot, multiarch
> /usr/include # affected by sysroot
>
> On Debian x86_64, `/usr/include/x86_64-linux-gnu/` exists. Adding
> `x86_64-pc-linux-gnu` (an possible value of `LLVM_DEFAULT_TARGET_TRIPLE`) may
> add more confusion.
This was already discussed pretty extensively in the past, but I'll try to give
a summary.
In the case of GCC, compiler can only compile binaries for a single target
that's set at configuration time. For example if you configured GCC with
`--target=x86_64-acme-linux-gnu`, the build links the binary as
`x86_64-acme-linux-gnu-gcc` and that binary would look for headers in
`/usr/include/x86_64-acme-linux-gnu` and libraries in
`/usr/lib/x86_64-acme-linux-gnu` (among other paths) so there's little
ambiguity.
In the case of Clang, the situation is a bit more difficult because a single
compiler supports multiple targets via `--target=` option and there are
different ways to spell the same target triple. For example, `x86_64-linux-gnu`
and `x86_64-unknown-linux-gnu` are considered equivalent, the latter is a
normalized version of the former (since you can omit emit components). The
question then is where to look for headers and libraries. Currently, we always
look in two locations: (1) we use `--target=<triple` as specified by the user
and (2) normalized version of that target. For example if the user passes
`--target=x86_64-linux-gnu`, we would first check
`<prefix>/lib/x86_64-linux-gnu` and then
`<prefix>/lib/x86_64-unknown-linux-gnu`.
In Fuchsia, we always prefer using normalized triples because that's less error
prone. For example, in the past we would install libraries in
`<prefix>/lib/x86_64-fuchsia` which works fine if you invoke the compiler with
`--target=x86_64-fuchsia`, but if you instead use
`--target=x86_64-unknown-fuchsia` (which some build systems would do), compiler
would fail to find libraries because it doesn't know how to denormalize the
triple. Using normalized triples addresses the issue because we can always
normalize the triple to get a canonical spelling.
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https://reviews.llvm.org/D89013/new/
https://reviews.llvm.org/D89013
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