Commits on May 30, 2017

1. ARMv5 needs +strict-align …

   Without that flag, LLVM generates unaligned memory access instructions, which are not allowed on ARMv5.
   
   For example, the 'hello world' example from `cargo --new` failed with:
   ```
   $ ./hello
   Hello, world!
   thread 'main' panicked at 'assertion failed: end <= len', src/libcollections/vec.rs:1113
   note: Run with `RUST_BACKTRACE=1` for a backtrace.
   ```
   
   I traced this error back to the following assembler code in `BufWriter::flush_buf`:
   ```
       6f44:       e28d0018        add     r0, sp, rust-lang#24
   [...]
       6f54:       e280b005        add     fp, r0, rust-lang#5
   [...]
       7018:       e5cd001c        strb    r0, [sp, rust-lang#28]
       701c:       e1a0082a        lsr     r0, sl, rust-lang#16
       7020:       03a01001        moveq   r1, rust-lang#1
       7024:       e5cb0002        strb    r0, [fp, rust-lang#2]
       7028:       e1cba0b0        strh    sl, [fp]
   ```
   
   Note that `fp` points to `sp + 29`, so the three `str*`-instructions should fill up a 32bit - value at `sp + 28`, which is later used as the value `n` in `Ok(n) => written += n`. This doesn't work on ARMv5 as the `strh` can't write to the unaligned contents of `fp`, so the upper bits of `n` won't get cleared, leading to the assertion failure in Vec::drain.
   
   With `+strict-align`, the code works as expected.

   

   jannic authored May 30, 2017
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