[LLVMdev] Why can't comparisons with negative zero be simplified?

[Sean Silva]

It seems like -0 should be fine.


-- Sean Silva

On Mon, Jul 6, 2015 at 2:13 PM, Robison, Arch <arch.robison at intel.com>
wrote:

>  In InstCombineCompares.cpp, routine InstCombiner::FoldFCmp_IntToFP_Cst,
> there are these lines:
>
>
>
>   // Comparisons with zero are a special case where we know we won't lose
>
>   // information.
>
>   bool IsCmpZero = RHS.isPosZero();
>
>
>
>   // If the conversion would lose info, don't hack on this.
>
>   if ((int)InputSize > MantissaWidth && !IsCmpZero)
>
>     return nullptr;
>
>
>
> Why check for positive zero instead of checking for any kind of zero?  My
> reading of IEEE 754-2008 is that floating-point comparison operations
> cannot distinguish a negative zero from a positive zero.  Further
> supporting this is that fact that http://llvm.org/docs/LangRef.html
> <https://urldefense.proofpoint.com/v2/url?u=http-3A__llvm.org_docs_LangRef.html&d=AwMFAg&c=8hUWFZcy2Z-Za5rBPlktOQ&r=Mfk2qtn1LTDThVkh6-oGglNfMADXfJdty4_bhmuhMHA&m=ZaWNNVon_US-YjLoXztqD5ve4xvIIhIXYdnk3Xqwz8w&s=xonYObEo1TXUyjK3RYC4Pg3kAaSvaYy_Lnvaiq5G1N0&e=>
> describes the difference between “ordered” and “unordered” as pertaining to
> QNAN operands,
>
> with no mention of negative zero.
>
>
>
> I tried fixing the issue, but then the following test in
> cast-int-fcmp-eq-0.ll fails:
>
>
>
> ; CHECK-LABEL: @i32_cast_cmp_oeq_int_n0_uitofp(
>
> ; CHECK: uitofp
>
> ; CHECK: fcmp oeq
>
> define i1 @i32_cast_cmp_oeq_int_n0_uitofp(i32 %i) {
>
>   %f = uitofp i32 %i to float
>
>   %cmp = fcmp oeq float %f, -0.0
>
>   ret i1 %cmp
>
> }
>
>
>
> Is this test really justified, or is it just reinforcing an oversight?
>
>
>
> - Arch D. Robison
>
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