add __breakpoint

[japaric]

this is an ARM compiler specific intrinsic which can be used to implement
CMSIS's __BKPT intrinsic

r? @gnzlbg

[gnzlbg]

Modulo the documentation nitpicks this LGTM.

[gnzlbg]

This should also be removed since it currently won't work and can be added later once it works in a backwards compatible way.

[gnzlbg]

Ah sorry, I see you also updated the Note below. So I think this is fine. I'll merge once CI is green. Let me know if there are any spurious CI failures.

[gnzlbg]

This is failing on aarch64 with:

error: <inline asm>:1:2: error: unrecognized instruction mnemonic
        BKPT 0
        ^
error: aborting due to previous error

We might need a cfg on target arch and use a different instruction there.

[paoloteti]

We might need a cfg on target arch and use a different instruction there.

If I'm not wrong is BKPT in aarch32 and BRK in aarch64.

[andre-richter]

Yes, its BRK

http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0802a/BRK.html

[alexcrichton]

Should this macro perhaps panic if any of the upper bits above 0xff are set? (I think right now it silently masks them off)

[japaric]

error: cannot find macro `panic!` in this scope
  --> crates/coresimd/src/../../../coresimd/arm/armclang.rs:43:5
   |
43 |     assert!(val >= 0 && val <= 255);
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
error: aborting due to previous error

Interesting.

Thinking more about it: Could we omit the assert! and document the behavior of % 256? I would prefer to avoid panicking branches in intrinsics as they can pull in the formatting machinery (few KBs) in unoptimized programs.

[gnzlbg]

Should this macro perhaps panic if any of the upper bits above 0xff are set? (I think right now it silently masks them off)

@alexcrichton it appears that it is not possible right now as @japaric mentioned. I've taken over this PR and just document that the only accepted values of val are in range [0, 255], and that values out-of-range are undefined behavior. In the implementation, I've documented the behavior of the constify macro on out-of-range values but that's an implementation detail that we can change however we want.