umaal instruction

[pftbest]

So I was playing with compiler-builtins-snapshot in qemu, and my program was hanging for no apparent reason. Then I looked into the code that was causing problems and found this instruction:

0x3c8a <core::num::flt2dec::strategy::grisu::format_shortest_opt+652>   umaal  r1, r5, r2, r3

This instruction should not be used on thumbv7m-none-eabi target.

[japaric]

umaal

This is a Cortex-M4 or newer instruction and shouldn't be generated by the thumbv7m target.

Do you have steps to reproduce? This sounds like the built-in thumbv7m target could be missing a "negative" LLVM feature. Are you passing any extra compiler flag related to codegen, like -C cpu=? Are you linking to C code?

[pftbest]

you can compile the code in question, it is on https://github.com/pftbest/rplayground

[japaric]

The instruction doesn't seem to come from compiler-builtins-snapshot AFAICT:

$ cargo clone compiler-builtins-snapshot && cd $_

$ xargo build --target thumbv7m-none-eabi

$ arm-none-eabi-objdump -CD target/thumbv7m-none-eabi/debug/libcompiler_builtins_snapshot.rlib | grep uma || echo no matches
no matches

$ xargo build --target thumbv7m-none-eabi --release

$ arm-none-eabi-objdump -CD target/thumbv7m-none-eabi/release/libcompiler_builtins_snapshot.rlib | grep uma || echo no matches
no matches

[japaric]

I see you have two .json files in your project. Could you, by mistake, by using the stm32f429zi target with QEMU instead of the qemu target? The former is a thumbv7em target and its binaries would have the umaal instruction.

[pftbest]

Could you, by mistake by using the stm32f429zi

no, it is commented out in the Makefile, I always use make clean make all and make run to build and run the project.

The instruction doesn't seem to come from compiler-builtins-snapshot

This may be the case, I'm not sure where it comes from, maybe it is from libcore, as is suggested by the core::num::flt2dec::strategy::grisu::format_shortest_opt prefix

[japaric]

Yes, they are coming from libcore. This is a bug in the target definition and should be fixed upstream, in rust-lang/rust.

[japaric]

Filed rust-lang/rust#37227

[pftbest]

Thank you