xbuild ignores .cargo/config for sysroot crates.

[andre-richter]

Hi,

for the following description please refer to an example uploaded to https://github.com/rust-embedded/rust-raspi3-tutorial/tree/no_float/float_reg_emission

This example should be buildable with a recent nightly rust and just executing make.

The project uses the aarch64-unknown-none target, and has additional rustflags in .cargo/config. However, it seems that xbuild is ignoring the contents of .cargo/config when it builds the sysroot crates.

In the example uploaded, it is set that no hard-float code should be emitted (-fp-armv8). This is honored for the actual crate being compiled, but not for the sysroot.

Here's a dissasembly of the emitted code where you can see that with xbuild, an ARM floating point register (q0) is being emitted:

core::panicking::panic_fmt::h7472082024a4b065:
   80218:       20 00 c0 3d     ldr     q0, [x1]
   8021c:       28 08 40 f9     ldr     x8, [x1, #0x10]
   80220:       09 00 00 b0     adrp    x9, #0x1000
   80224:       0a 00 00 b0     adrp    x10, #0x1000
   80228:       29 a1 15 91     add     x9, x9, #0x568
   8022c:       4a a1 15 91     add     x10, x10, #0x568
   80230:       e9 2b bd a9     stp     x9, x10, [sp, #-0x30]!
   80234:       e0 0b 00 f9     str     x0, [sp, #0x10]
   80238:       e0 03 00 91     mov     x0, sp
   8023c:       e0 83 81 3c     stur    q0, [sp, #0x18]
   80240:       e8 17 00 f9     str     x8, [sp, #0x28]
   80244:       58 00 00 94     bl      #0x160 <rust_begin_unwind>
   80248:       20 00 20 d4     brk     #0x1

Using xargo within the same example, this doesn't happen:

xargo build --target=aarch64-unknown-none --release
cp target/aarch64-unknown-none/release/kernel8 .
make objdump

No float regs here:

core::panicking::panic_fmt::h4f28b4483e5ece01:
   80224:       08 00 00 b0     adrp    x8, #0x1000
   80228:       08 21 16 91     add     x8, x8, #0x588
   8022c:       e8 0f 1d f8     str     x8, [sp, #-0x30]!
   80230:       0a 00 00 b0     adrp    x10, #0x1000
   80234:       4a 21 16 91     add     x10, x10, #0x588
   80238:       28 24 40 a9     ldp     x8, x9, [x1]
   8023c:       ea 83 00 a9     stp     x10, x0, [sp, #0x8]
   80240:       e0 03 00 91     mov     x0, sp
   80244:       2a 2c 42 29     ldp     w10, w11, [x1, #0x10]
   80248:       e8 a7 01 a9     stp     x8, x9, [sp, #0x18]
   8024c:       ea 2f 05 29     stp     w10, w11, [sp, #0x28]
   80250:       59 00 00 94     bl      #0x164 <rust_begin_unwind>
   80254:       20 00 20 d4     brk     #0x1

[andre-richter]

I was on an older version of xbuild. The issue has already been detected and fixed via #25 👍

[phil-opp]

Thanks for the update!