[llvm-dev] LLVM behavior different depending on function symbol name

[Tom Stellard via llvm-dev]

On 06/19/2017 11:45 AM, Andrew Kelley via llvm-dev wrote:
> Greetings,
> 
> I have a Zig implementation of ceil which is emitted into LLVM IR like this:
> 
> ; Function Attrs: nobuiltin nounwind
> define internal fastcc float @ceil(float) unnamed_addr #3 !dbg !644 {
> Entry:
>   %x = alloca float, align 4
>   store float %0, float* %x
>   call void @llvm.dbg.declare(metadata float* %x, metadata !649, metadata !494), !dbg !651
>   %1 = load float, float* %x, !dbg !652
>   %2 = call fastcc float @ceil32(float %1) #8, !dbg !656
>   ret float %2, !dbg !657
> }
> 

What does the declaration of @ceil32() look like ?


> Test case:
> 
> test "math.ceil" {
>     assert(ceil(f32(0.0)) == ceil32(0.0));
>     assert(ceil(f64(0.0)) == ceil64(0.0));
> }
> 
> 
> When I compile with optimizations on, this test case fails. The optimized code for the test case ends up being a call to panic (assertion failure), which means that LLVM determined the test failed at compile-time.
> 
> What's strange about this is that if I change the function name from @ceil to @ceil_asdf (and change the callers) then the test passes.
> 
> So I think LLVM is doing some kind of string comparison on the symbol name and detecting that it is "ceil" and then having different, undesired behavior.
> 
> I tried putting `nobuiltin` in the function attributes and at the callsite, but that did not change anything.
> 
> Any ideas what's going on?
> 
> Downstream issue: https://github.com/zig-lang/zig/issues/393
> 
> Regards,
> Andrew
> 
> 
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