We run:

$ clang hello.c -pipe -Wa,- &
$ sudo cat /proc/pgrep bin/as | tail -n1/stack

[<0>] wait_woken+0x43/0x80
[<0>] n_tty_read+0x42f/0x880
[<0>] tty_read+0x7b/0xe0
[<0>] vfs_read+0x89/0x130
[<0>] ksys_read+0x52/0xc0
[<0>] do_syscall_64+0x55/0x110
[<0>] entry_SYSCALL_64_after_hwframe+0x44/0xa9
[<0>] 0xffffffffffffffff

so I assume the assembler is waiting maybe for an EOF to be written. Let me see if I can find that in the source of GAS.

Looks like EOF is meant for checking read values, but it itself is not writeable. It seems that closing a pipe is the better way to signal this?

I'm getting the feeling we should pick up -Wa,- in clang/lib/Driver/ToolChains/Gnu.cpp#tools::gnutools::Assembler::ConstructJob

and then somehow in llvm/lib/Support/Program.cpp#sys::ExecuteAndWait not do the wait. Or find a way to close the fd once we've finished all output?

It seems that clang will always write the assembly to a tempfile, then invoke the assembler on that temp file. So it seems that the only way for Clang to pipe the assembly into the assembler via stdout/stdin is if clang supported -pipe.

While Clang documents support for -pipe.
https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-pipe
Looks like Clang eats -pipe:
https://github.com/llvm-mirror/clang/blob/391667a023f79287f9c40868f34f08c161555556/lib/Driver/Driver.cpp#L962
commit r110007 has the log:
Driver: Start ripping out support for -pipe, which is worthless
and complicates
too many other things.

So maybe the right thing to do is to filter out -Wa,- from the CmdArgs for constructing the assembler job, maybe with a comment that that should be removed should -pipe ever be reimplemented?

Looks like -pipe -Wa,- doesn't buy us any speedup (maybe why -pipe was dropped from clang), so we'll just remove the use of them from the kernel.
https://lkml.org/lkml/2018/10/24/111

Probably should still filter out -Wa,- and NOT pass it to the assembler.