Description
I'm helping to build a small operating system to run on Cortex-M0 processors. I'm compiling for the thumbv6m-none-eabi target. When using the compiler builtin function __aeabi_lmul it seems that the compiler is passing in the arguments to the wrong registers. For example, this piece of code:
fn do_mul(a: isize, b: isize) -> isize {
a * b
}Gets compiled down to:
0: b580 push {r7, lr}
2: af00 add r7, sp, #0
4: b088 sub sp, #32
6: 460a mov r2, r1
8: 4603 mov r3, r0
a: 9004 str r0, [sp, #16]
c: 9105 str r1, [sp, #20]
e: 9203 str r2, [sp, #12]
10: 9302 str r3, [sp, #8]
12: e7ff b.n 14 <lmul_test::do_a_mul::hb8c09fc6501fea17+0x14>
14: 9804 ldr r0, [sp, #16]
16: 9006 str r0, [sp, #24]
18: 9805 ldr r0, [sp, #20]
1a: 9007 str r0, [sp, #28]
1c: 9906 ldr r1, [sp, #24]
1e: 17c2 asrs r2, r0, #31
20: 17cb asrs r3, r1, #31
22: 9001 str r0, [sp, #4]
24: 4608 mov r0, r1
26: 4611 mov r1, r2
28: 9a01 ldr r2, [sp, #4]
2a: f7ff fffe bl 0 <__aeabi_lmul>
...
According the the ARM ABI Documentation (Section 5.1.1.1), passing a 64bit value as an argument should store them in registers r0 and r1 or r2 and r3 as a pair. With the low bits being stored in r0/2 and the high bits stored in r1/3.
Looking at the dissasembled code, the first argument, which was initially stored in r0 gets its low and high bits split into r0 and r3 respectively, not the expected r0 and r1. The second argument also gets split into r2 and r1 for its low and high bits. So it appears that the high bits for the two values have been swapped.
Because the values are sign extended from 32 bits, this is only an issue if the signs of the two arguments differ. If they're the same the high bit registers will happen to be the same. But when the signs differ the function returns a wrong result in the r1 register. This causes a panic if overflow checking is enabled.
I've not noticed this issue anywhere else, but it could possibly be hidden around in some of the other compiler builtins.
I've created a sample repo here that shows off the bug. You'll need an ARM toolchain to actually compile it down to a binary, but you should still be able to cross compile it into a static library. (the command I was using was xargo build --target=thumbv6m-none-eabi)
Version info:
lmul-test|master ⇒ rustc --version --verbose
rustc 1.16.0-nightly (47c8d9fdc 2017-01-08)
binary: rustc
commit-hash: 47c8d9fdcf2e6502cf4ca7d7f059fdc1a2810afa
commit-date: 2017-01-08
host: x86_64-apple-darwin
release: 1.16.0-nightly
LLVM version: 3.9