LLVM assertion when compiling `core` for the MSP430 architecture

[japaric]

STR

$ cat msp430.json
{
  "arch": "msp430",
  "data-layout": "e-m:e-p:16:16-i32:16:32-a:16-n8:16",
  "executables": true,
  "linker": "msp430-elf-gcc",
  "llvm-target": "msp430",
  "max-atomic-width": 0,
  "no-integrated-as": true,
  "os": "none",
  "panic-strategy": "abort",
  "relocation-model": "static",
  "target-endian": "little",
  "target-pointer-width": "16"
}

$ rustup component add rust-src

$ rustc --target msp430 $(rustc --print sysroot)/lib/rustlib/src/rust/src/libcore/lib.rs
rustc: /buildslave/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/llvm/lib/Support/APInt.cpp:939: llvm::APInt llvm::APInt::trunc(unsigned int) const: Assertion `width < BitWidth && "Invalid APInt Truncate request"' failed.

Meta

$ rustc -V
rustc 1.15.0-nightly (43006fcea 2016-11-15)

I didn't see this when I tested #37672 because the default rustbuild configuration has LLVM assertions disabled whereas nightlies enable the assertions. 😞

cc @alexcrichton

[alexcrichton]

We've wondered from time to time whether to turn LLVM assertions on during development, but most of the time we conclude "no" as it slows down builds and it's quite rare for them to trigger during typical development.

Should write down somewhere that when adding a new target it's highly recommended to do so!

[pftbest]

Minimal IR that triggers this assertion:
bugpoint-reduced-simplified.ll

$ llc bugpoint-reduced-simplified.ll 
Assertion failed: (width < BitWidth && "Invalid APInt Truncate request"), function trunc, file /Users/vadzim/llvm/lib/Support/APInt.cpp, line 917.
Stack dump:
0.  Program arguments: llc bugpoint-reduced-simplified.ll 
1.  Running pass 'Function Pass Manager' on module 'bugpoint-reduced-simplified.ll'.
2.  Running pass 'MSP430 DAG->DAG Pattern Instruction Selection' on function '@_ZN4core3num6bignum5tests6Big8x37div_rem17h4a91efdf5fc031b4E'
Abort trap: 6

[shepmaster]

Looks very similar to an issue that AVR ran into. I've had this patch that I've been applying manually, not sure how valid it is to upstream.

[pftbest]

@shepmaster
I don't think this is a good solution to just hide the symptoms, the real issue will remain, and probably cause more harm in the future.

I did some digging and found out that in this case umul gets legalized incorrectly, and in result zero extend from i16 to i16 is generated. And such zero extends should not exist in DAG, thats why this assertion fires.

[shepmaster]

I don't think this is a good solution to just hide the symptoms

Absolutely, which is why I haven't submitted it anywhere; I was simply trying to help. I apologize.

I would like to see what changes you make to LLVM to fix this, as it's likely AVR will need the same thing. You might even fix it, now that AVR is in LLVM :-).

[pftbest]

I was simply trying to help. I apologize.

I didn't mean to be rude. I'm sorry if it sounded that way.

I would like to see what changes you make to LLVM to fix this, as it's likely AVR will need the same thing.

Of course, I will try fix this, but I'm still new to LLVM so can't promise anything. Maybe aKor from #llvm (author of MSP430 backend) will help, he already knows about this issue.

[pftbest]

I have posted the fix for review:
https://reviews.llvm.org/D27154
It fixes the assertion, but the resulting assembly code looks very bad, possibly incorrect, so I think more work is needed still.

[pftbest]

Fixed in llvm upstream, commit rL288857.
Don't know if I should backport it to rust-llvm, or should we wait for llvm 4?

[japaric]

@pftbest I'd say backport if it's easy to do so. No idea how long the next llvm-up will take.