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.