Multiplication instructions generated for target that does not support them

[Rahix]

When compiling the following code:

#![no_std]
#![no_main]
#![feature(lang_items, panic_handler)]

#[no_mangle]
pub extern fn main() {
    unsafe {
        let x: u8 = core::ptr::read_volatile(42 as *mut u8);
        let y = x * 13;
        core::ptr::write_volatile(42 as *mut u8, y);
    }
}

pub mod std {
    #[lang = "eh_personality"]
    pub unsafe extern "C" fn rust_eh_personality(
        _state: (),
        _exception_object: *mut (),
        _context: *mut (),
    ) -> () {
    }

    #[panic_handler]
    fn panic(_info: &::core::panic::PanicInfo) -> ! {
        loop { }
    }
}

using rustc --target=avr-attiny85 main.rs --emit=asm and the following target:

{
  "llvm-target": "avr-unknown-unknown",
  "cpu": "attiny85",
  "target-endian": "little",
  "target-pointer-width": "16",
  "target-c-int-width": "16",
  "os": "unknown",
  "target-env": "",
  "target-vendor": "unknown",
  "arch": "avr",
  "data-layout": "e-P1-p:16:8-i8:8-i16:8-i32:8-i64:8-f32:8-f64:8-n8-a:8",

  "executables": true,

  "linker": "avr-gcc",
  "linker-flavor": "gcc",
  "pre-link-args": {
    "gcc": ["-Os", "-mmcu=attiny85"]
  },
  "exe-suffix": ".elf",
  "post-link-args": {
    "gcc": ["-Wl,--gc-sections"]
  },

  "singlethread": true,
  "no-builtins": false,

  "no-default-libraries": false
}

LLVM emits a mul instruction, even though they are not supported on attiny85.

Unfortunately I don't have llvm tools available right now, to reduce the IR. I guess this is the responsible line:

  %1 = call addrspace(1) { i8, i1 } @llvm.umul.with.overflow.i8(i8 %0, i8 13)

[dylanmckay]

Here's a reproduction

declare {i8, i1 } @llvm.umul.with.overflow.i8(i8, i8) 

define {i8, i1} @test(i8 %a) {
  %1 = call addrspace(1) { i8, i1 } @llvm.umul.with.overflow.i8(i8 %a, i8 13) 
  ret {i8, i1} %1
}

Compile with

./bin/llc -march=avr -mcpu=attiny85 test.ll -o -  

Outputs

test:                                   ; @test
; %bb.0:
        movw    r26, r24
        ldi     r24, 13
        mul     r22, r24 ; HERE'S THE MUL
        mov     r25, r1
        clr     r1
        st      X+, r0
        ldi     r24, 1
        cpi     r25, 0
        brne    LBB0_2
; %bb.1:
        ldi     r24, 0
LBB0_2:
        andi    r24, 1
        st      X, r24
        ret
.Lfunc_end0:
        .size   test, .Lfunc_end0-test
                                        ; -- End function

[aykevl]

I think I've seen a mul without the llvm.umul.with.overflow.i8 intrinsic, but I'm not sure. I should make a repro out of that. Noting it here in case it is relevant.

[dylanmckay]

Fixed in upstream LLVM at llvm-project/llvm-project-20170507@a684637f8b667527882ac4f50ea9ecb482e248aa.

[dylanmckay]

Cherry-picked in a3b0834