ARMv5 needs +strict-align

[jannic]

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, #24
[...]
    6f54:       e280b005        add     fp, r0, #5
[...]
    7018:       e5cd001c        strb    r0, [sp, #28]
    701c:       e1a0082a        lsr     r0, sl, #16
    7020:       03a01001        moveq   r1, #1
    7024:       e5cb0002        strb    r0, [fp, #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.

[rust-highfive]

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[eddyb]

r? @japaric

[japaric]

Thanks, @jannic. That makes sense. The thumbv6m-none-eabi needed the same treatment. Makes me wonder why LLVM is not setting that option by default for these subarchitectures.

@bors r+

[bors]

📌 Commit 4450807 has been approved by japaric

[jannic]

According to rust-lang/llvm@4fb2f33, "clang is now responsible for setting strict-align". The same is probably true for rustc.