reproducable wasm32 bug

[gilescope]

I'm really sorry if this isn't a compiler bug, but it smells like one. I've managed to bag it into a few lines of code. Change or reorder anything and the test will pass.

If you run cargo test this repo it will pass:

https://github.com/gilescope/wasm-repro1.git

If you add in --target=wasm32-unknown-unknown it will fail. ( You will need to cargo install wasm-bindgen-cli)
It seems to correctly figure out that the condion is false but then take the true branch of the if anyway. There's no unsafe in the code so there should be no UB....

#[wasm_bindgen_test]
#[test]
fn test() {
    let s: &[u8] = &[1];
    let masked = s[0];
    // The below cond should evaluate to false but,
    // what seems to happen is even though it evaluates to false,
    // the if takes the wrong branch!!!!
    let cond = s[0] as u32 == 0; //should evaluate to false.

    // Comment out line below to pass the test on --target=wasm32-unknown-unknown ????
    let m1 = masked << 0;

    assert_eq!(Err(()), if cond {
        Ok(3333)
    } else {
        //Should go here.
        Err(())
    });
}

Why for instance does << 0 have to be there for the bug to manifest itself? I am clueless...

This is with the latest nightly ( rustc 1.54.0-nightly (5dc8789 2021-05-21) ), but have confirmed it fails on beta too.

I would love to know what is going on - it seems very wierd behaviour.

[apiraino]

Assigning priority as discussed in the Zulip thread of the Prioritization Working Group.

@rustbot label -I-prioritize +P-high

[RalfJung]

Smells like an LLVM miscompilation issue? Since it is target-specific, a MIR issue seems unlikely. Cc @rust-lang/wg-llvm

[apiraino]

I'm afraid the ping didn't work

@rustbot ping @rust-lang/wg-llvm

cc: @cuviper @nikic @nagisa

[rustbot]

Error: This team (@rust-lang/wg-llvm) cannot be pinged via this command; it may need to be added to triagebot.toml on the master branch.

Please let @rust-lang/release know if you're having trouble with this bot.

[rustbot]

Error: This team (@rust-lang/wg-llvm~) cannot be pinged via this command; it may need to be added to triagebot.toml on the master branch.

Please let @rust-lang/release know if you're having trouble with this bot.

[nikic]

target datalayout = "e-m:e-p:32:32-i64:64-n32:64-S128-ni:1:10:20"
target triple = "wasm32-unknown-unknown"

define i32 @test(i8* %p, i8* %p2) {
  %v = load i8, i8* %p
  %v.ext = zext i8 %v to i32
  %cond = icmp eq i32 %v.ext, 0
  %shl = shl i8 %v, 0
  store i8 %shl, i8* %p2
  br label %bb2

bb2:
  br i1 %cond, label %bb3, label %bb4

bb4:
  ret i32 0

bb3:
  ret i32 1
}

With llc -O0

	.text
	.file	"test339.ll"
	.section	.text.test,"",@
	.globl	test                            # -- Begin function test
	.type	test,@function
test:                                   # @test
	.functype	test (i32, i32) -> (i32)
	.local  	i32, i32, i32, i32
# %bb.0:
	local.get	0
	i32.load8_u	0
	local.set	2
	local.get	2
	i32.eqz
	drop
	local.get	1
	local.get	2
	i32.store8	0
# %bb.1:                                # %bb2
	block   	
	local.get	3
	i32.eqz
	br_if   	0                               # 0: down to label0
# %bb.2:                                # %bb4
	i32.const	0
	local.set	4
	local.get	4
	return
.LBB0_3:                                # %bb3
	end_block                               # label0:
	i32.const	1
	local.set	5
	local.get	5
	return
	end_function
.Lfunc_end0:
	.size	test, .Lfunc_end0-test
                                        # -- End function

Note that local.get 3 reads a local that has not been initialized.

[nikic]

Upstream issue: https://bugs.llvm.org/show_bug.cgi?id=51651