Add new rustc target for Arm64 machines that can target the iphonesimulator

[deg4uss3r]

This PR lands a new target (aarch64-apple-ios-sim) that targets arm64 iphone simulator, previously unreachable from Apple Silicon machines.

resolves #81632

r? @shepmaster

[badboy]

tl;dr: I finally made this work with my static libraries to work inside the iOS simulator on an M1 macbook pro. I had to use the triple arm64-apple-ios7.0.0-simulator to make it behave.

heads up: This is based on #81451 to get LLVM 12. I have not tested yet if that's required.

Details on how I got there:

First I looked at what the difference is between C code compiled for arm64 iOS and for arm64 iOS Simulator:

❯ cat t.c
#include <stdio.h>

int foo()
{
  printf("hello world\n");
  return 0;
}

❯ clang -target arm64-apple-ios-simulator t.c -c -o t-sim.o -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator.sdk
❯ clang -target arm64-apple-ios t.c -c -o t-ios.o -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS.sdk
❯ otool -l t-sim.o | rg 'VERSION|platform|version'
      cmd LC_BUILD_VERSION
 platform 7
❯ otool -l t-ios.o | rg 'VERSION|platform|version'
      cmd LC_VERSION_MIN_IPHONEOS
  version 7.0

As you can see the simulator one uses LC_BUILD_VERSION, whereas the iOS one uses LC_VERSION_MIN_IPHONEOS.

After a bunch of tries, trying to trace the rustc and LLVM code I was unable to fully determine how LLVM decides which way to use.
No matter what I did the objects falling out of rustc used LC_VERSION_MIN_IPHONEOS.

So I went back to look at the C code again.
This time comparing the LLVM IR.

Here's the relevant difference between t.c compiled with clang -emit-llvm for iOS or iOS-simulator:

❯ diff -u t-ios.ll t-sim.ll  | head -8
--- t-ios.ll	2021-02-12 11:10:21.000000000 +0100
+++ t-sim.ll	2021-02-12 11:10:03.000000000 +0100
@@ -1,7 +1,7 @@
 ; ModuleID = 't.c'
 source_filename = "t.c"
 target datalayout = "e-m:o-i64:64-i128:128-n32:64-S128"
-target triple = "arm64-apple-ios7.0.0"
+target triple = "arm64-apple-ios7.0.0-simulator"

Low and behold: the triple has the version in it.

So I swapped out the triple in aarch64_apple_ios_sim.rs, recompiled my compiler, then used that to build my test library:

❯ cat src/lib.rs
#[no_mangle]
pub extern "C" fn hello_world() -> u32 {
    1+1
}
❯ rustc +stage1 --target aarch64-apple-ios-sim src/lib.rs --crate-type staticlib
❯ otool -l liblib.a | rg 'VERSION|platform|version' | head -2
      cmd LC_BUILD_VERSION
 platform 7

There we go!

Now it turns out: for macOS we do the same: we put the deployment target version into the triple:

rust/compiler/rustc_target/src/spec/apple_base.rs

Lines 57 to 65 in a118ee2

	fn macos_deployment_target() -> (u32, u32) {
	let deployment_target = env::var("MACOSX_DEPLOYMENT_TARGET").ok();
	let version = deployment_target
	.as_ref()
	.and_then(|s| s.split_once('.'))
	.and_then(|(a, b)| a.parse::<u32>().and_then(|a| b.parse::<u32>().map(|b| (a, b))).ok());
	version.unwrap_or((10, 7))
	}

I guess we need to start doing that for iOS too? I can work on integrating that into the code of this PR.
(Side note: I took my rustc build to my iOS project and I can now run tests using the simulator, there's more work ahead on our side because of course we can't build one fat library anymore for hardware and simulator targets ...)

[deg4uss3r]

Awesome detective work @badboy !!!!

Looks like we'll need LLVM12 still I tried building against the latest master with the triple changed to arm64-apple-ios7.0.0-simulator

and I'm still getting:

Showing All Messages
In /Users/rth/work/core/target/aarch64-apple-ios-sim/debug/libcore.a(sqlite3.o), building for iOS Simulator, but linking in object file built for iOS, for architecture arm64

[badboy]

Awesome detective work @badboy !!!!

Looks like we'll need LLVM12 still I tried building against the latest master with the triple changed to arm64-apple-ios7.0.0-simulator

and I'm still getting:

Showing All Messages
In /Users/rth/work/core/target/aarch64-apple-ios-sim/debug/libcore.a(sqlite3.o), building for iOS Simulator, but linking in object file built for iOS, for architecture arm64

Well, that one might be because sqlite3 is built with the wrong target. If sqlite3 is built using the cc crate you might need a export CFLAGS_aarch64_apple_ios_sim="-target arm64-apple-ios7.0.0-simulator" (or TARGET_CFLAGS="...") to make it work.

I should fine some time tomorrow to extend this PR with the necessary steps and also look into cc and whether it needs changes to make it work out of the box.

[deg4uss3r]

💯 adding that flag to my build script works I can run my app, this is awesome @badboy 😃

I'll let you add your commits as I am sure it's much cleaner than my branch is now lol

[badboy]

not sure if you noticed, but I sent a PR to your branch here: deg4uss3r#1
If you merge that it should show up here and from there it can get proper review.

[shepmaster]

Seems believable, and it sounds like two separate people got it to work, so that's about as good of testing as I'd expect for this level.

Would you mind squashing the commits (don't really need the intermediate "fix formatting") and then we can merge!

[deg4uss3r]

@shepmaster ungracefully done!

[LingMan]

@deg4uss3r You can squash the typo fix, too. I really don't care about credit for something that minor.

[shepmaster]

@bors r+ rollup

Thanks!

[bors]

📌 Commit 8d6ad11 has been approved by shepmaster