Binary size regression on msp430 due to #98582

[cr1901]

I recently had CI start failing for some firmware I use to make sure Rust still compiles msp430 code correctly. It appears that as of #98582, Rust has stopped optimizing out dead panic code; there is no I/O device to send the panic strings to, and so panic strings- and setting up arguments to access panic strings- should be considered dead code.

Normally, the firmware size should take 1994 (1992+2) bytes:

cargo build --release -Zbuild-std=core --target=msp430-none-elf
msp430-elf-size target/msp430-none-elf/release/at2xt
   text    data     bss     dec     hex filename
   1992       2      48    2042     7fa target/msp430-none-elf/release/at2xt

However, the firmware size had gone up 100 bytes (2048+46 = 2094- 5% regression) before #98582 was reverted by #99495:

cargo build --release -Zbuild-std=core --target=msp430-none-elf
error: linking with `msp430-elf-gcc` failed: exit status: 1
  |
  = note: "msp430-elf-gcc" "/home/william/Projects/embedded/msp430/AT2XT/target/msp430-none-elf/release/deps/at2xt-4706a59318f01763.at2xt.aa970193-cgu.0.rcgu.o" "-L" "/home/william/Projects/embedded/msp430/AT2XT/target/msp430-none-elf/release/deps" "-L" "/home/william/Projects/embedded/msp430/AT2XT/target/release/deps" "-L" "/home/william/Projects/embedded/msp430/AT2XT/target/msp430-none-elf/release/build/msp430-rt-587a8251c4c89199/out" "-L" "/home/william/Projects/embedded/msp430/AT2XT/target/msp430-none-elf/release/build/msp430g2211-1199852debc08ead/out" "-L" "/home/william/Projects/toolchains/build-llvm-toolchain/build-rust/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/msp430-none-elf/lib" "-Wl,-Bstatic" "/tmp/rustcy1Mond/libmsp430_rt-19edab0518267d0f.rlib" "-Wl,--start-group" "-Wl,--end-group" "/home/william/Projects/embedded/msp430/AT2XT/target/msp430-none-elf/release/deps/libcompiler_builtins-1ef8666a5bfcc3f4.rlib" "-Wl,-Bdynamic" "-L" "/home/william/Projects/toolchains/build-llvm-toolchain/build-rust/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/msp430-none-elf/lib" "-o" "/home/william/Projects/embedded/msp430/AT2XT/target/msp430-none-elf/release/deps/at2xt-4706a59318f01763" "-nodefaultlibs" "-Tlink.x" "-mcpu=msp430" "-nostartfiles" "-lmul_none" "-lgcc"
  = note: /home/william/.local/bin/../lib/gcc/msp430-elf/9.2.0/../../../../msp430-elf/bin/ld: /home/william/Projects/embedded/msp430/AT2XT/target/msp430-none-elf/release/deps/at2xt-4706a59318f01763 section `.rodata' will not fit in region `ROM'
          /home/william/.local/bin/../lib/gcc/msp430-elf/9.2.0/../../../../msp430-elf/bin/ld: section .rodata VMA wraps around address space
          /home/william/.local/bin/../lib/gcc/msp430-elf/9.2.0/../../../../msp430-elf/bin/ld: section .vector_table LMA [000000000000ffe0,000000000000ffff] overlaps section .rodata LMA [000000000000ff48,000000000001000b]
          /home/william/.local/bin/../lib/gcc/msp430-elf/9.2.0/../../../../msp430-elf/bin/ld: region `ROM' overflowed by 46 bytes
          collect2: error: ld returned 1 exit status


error: could not compile `at2xt` due to previous error
error: Recipe `timer` failed on line 10 with exit code 101

I've created an MVCE from the failing CI to illustrate:

Code

Instructions

1. Make sure the msp430-elf-gcc toolchain is installed. Optionally install just for convenience.

2. git clone https://github.com/cr1901/msp430-size. Use commit e44cf66 specifically.

3. Compile rustc and add your new rustc to rustup using rustup toolchain link override-name /path/to/override. Use this override for both rustc commits in the next step.

4. Run the below command twice (using different target dirs):

   cargo +override-name rustc --manifest-path=./take-api/Cargo.toml --release -Zbuild-std=core --example=once -- --emit=obj=target/msp430-none-elf/release/examples/once.o,llvm-ir=target/msp430-none-elf/release/examples/once.ll,asm=target/msp430-none-elf/release/examples/once.s
   

   The above command needs to be run with two different compilers:

   • Once with a compiler before Allow destructuring opaque types in their defining scopes #98582 or after Revert "Rollup merge of #98582 - oli-obk:unconstrained_opaque_type, r… #99495.
   • Once with a compiler in between those PRs.
   • I used 4a742a6 and 03d488b, but the actual commits aren't essential.

5. Extra compiler artifacts can be generated like so:

   msp430-elf-objdump -Cd target/msp430-none-elf/release/examples/once
   msp430-elf-readelf -a --wide target/msp430-none-elf/release/examples/once
   msp430-elf-objdump -Cd target/msp430-none-elf/release/examples/once.o
   msp430-elf-readelf -a --wide target/msp430-none-elf/release/examples/once.o
   msp430-elf-size target/msp430-none-elf/release/examples/once
   

Expected Results

For both Rust compiler commits, I expected to see the following output from msp430-elf-size, with no panic strings emitted:

william@xubuntu-dtrain:~/Projects/embedded/msp430/msp430-size$ msp430-elf-size -A target/msp430-none-elf/release/examples/once
target/msp430-none-elf/release/examples/once  :
section              size    addr
.vector_table          32   65504
.text                 168   49152
.rodata                 4   49320
.bss                    2     512
.data                   0     514
.MSP430.attributes     23       0
Total                 229
strings target/msp430-none-elf/release/examples/once | grep called

Instead, the compiler which had PR #98582 had a different size, and a panic string for Option::unwrap() failure was emitted into the binary:

william@xubuntu-dtrain:~/Projects/embedded/msp430/msp430-size$ msp430-elf-size -A target/msp430-none-elf/release/examples/once
target/msp430-none-elf/release/examples/once  :
section              size    addr
.vector_table          32   65504
.text                 200   49152
.rodata                48   49352
.bss                    2     512
.data                   0     514
.MSP430.attributes     23       0
Total                 305
strings target/msp430-none-elf/release/examples/once | grep called
?called `Option::unwrap()` on a `None` value

Regression Commits

Based on git bisect, this regression was introduced by PR #98582. Specifically 728c7e8 introduced the regression. This regression was fixed by #99495. However, @oli-obk asked me to open an issue anyway.

@rustbot modify labels: +regression-from-stable-to-nightly -regression-untriaged

Other Context

Although I don't have numbers handy, this regression also affects libcores compiled with the panic_immediate_abort feature for less marked size regression.