[llvm] [Inliner] Add support for preserving `nocapture` param attr (PR #113418)

[via llvm-commits]

goldsteinn wrote:

> Can you remind me what the original motivating case was? Something like a nocapture wrapper around inline assembly?

Yes, currently there is no way to apply `nocapture` to the arguments of an assembly call.

For example:
```
#include <stdint.h>
#include <sys/syscall.h>
#include <unistd.h>
#include <linux/futex.h>

static long futex_wait(unsigned const __attribute__((noescape)) * mem,
                       unsigned val) {
  register long r_rax __asm__("rax") = SYS_futex;
  register unsigned const * r_rdi __asm__("rdi") = mem;
  register long r_rsi __asm__("rsi") = FUTEX_WAIT_PRIVATE;
  register unsigned r_rdx __asm__("rdx") = val;
  register long r_r10 __asm__("r10") = 0;
  __asm__ volatile("syscall"
                   : "+r"(r_rax)
                   : "r"(r_rdi), "r"(r_rsi), "r"(r_rdx), "r"(r_r10),
                     "m"(*((unsigned const(*)[1])r_rdi))
                   : "r11", "rcx");
  return r_rax;
}


long do_futex_wait(unsigned * mem) {
    //...
    futex_wait(mem, 1);
    //...
}
```

When `futex_wait` gets inlined into `do_futex_wait`, the `nocapture` will be lost forever and I don't think there is any way currently to get a `nocapture` on the `mem` argument to the inline-asm callsite.


https://github.com/llvm/llvm-project/pull/113418