tree 1297eb7d6d99f69b97246083dd00902a4f491020
parent a2cdb247e1de9a53a553e214bb4a9fb85717f852
author Elliott Hughes <enh@google.com> 1689639301 -0700
committer Elliott Hughes <enh@google.com> 1689794407 -0700

De-pessimize SigSetConverter usage.

While looking at the disassembly for the epoll stuff I noticed that this
expands to quite a lot of code that the compiler can't optimize out for
LP64 (because it doesn't know that the "copy the argument into a local
and then use the local" bit isn't important).

There are two obvious options here. Something like this:
```
  int signalfd64(int fd, const sigset64_t* mask, int flags) {
    return __signalfd4(fd, mask, sizeof(*mask), flags);
  }

  int signalfd(int fd, const sigset_t* mask, int flags) {
  #if defined(__LP64__)
    return signalfd64(fd, mask, flags);
  #else
    SigSetConverter set = {.sigset = *mask};
    return signalfd64(fd, &set.sigset64, flags);
  #endif
  }
```
Or something like this:
```
  int signalfd64(int fd, const sigset64_t* mask, int flags) {
    return __signalfd4(fd, mask, sizeof(*mask), flags);
  }

  #if defined(__LP64__)
  __strong_alias(signalfd, signalfd64);
  #else
  int signalfd(int fd, const sigset_t* mask, int flags) {
    SigSetConverter set = {};
    set.sigset = *mask;
    return signalfd64(fd, &set.sigset64, flags);
  }
  #endif
```
The former is slightly more verbose, but seems a bit more obvious, so I
initially went with that. (The former is more verbose in the generated
code too, given that the latter expands to _no_ code, just another symbol
pointing to the same code address.)

Having done that, I realized that slight changes to the interface would
let clang optimize away most/all of the overhead for LP64 with the only
preprocessor hackery being in SigSetConverter itself.

I also pulled out the legacy bsd `int` conversions since they're only
used in two (secret!) functions, so it's clearer to just have a separate
union for them. While doing so, I suppressed those functions for
riscv64, since there's no reason to keep carrying that mistake forward.

posix_spawn() is another simple case that doesn't actually benefit from
SigSetConverter, so I've given that its own anonymous union too.

Test: treehugger
Change-Id: Iaf67486da40d40fc53ec69717c3492ab7ab81ad6