[cfe-dev] int-to-bool conversion issues

[Stephen Canon via cfe-dev]

But: you can use ! Instead of ~ to avoid this issue.

– Steve

> On Sep 13, 2019, at 4:52 PM, Stephen Canon <scanon at apple.com> wrote:
> 
> The C and C++ language standards fully specify this behavior. There’s no flexibility here to change it.
> 
> – Steve
> 
>> On Sep 13, 2019, at 4:49 PM, Alexandre Isoard via cfe-dev <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>> wrote:
>> 
>> Sorry for the typo:
>> 
>> false → 0b0000 → 0b1111 → true
>> true → 0b0001 → 0b1110 → true
>> 
>> On Fri, Sep 13, 2019 at 1:38 PM Alexandre Isoard <alexandre.isoard at gmail.com <mailto:alexandre.isoard at gmail.com>> wrote:
>> Hello,
>> 
>> I encountered a really annoying issue, I am not sure it is Clang or C++ language issue.
>> 
>> But basically, the following code:
>> 
>> bool foo(bool a) {
>> return ~a;
>> }
>> 
>> Collapse into:
>> 
>> bool foo(bool a) {
>> return true;
>> }
>> 
>> The explanation is simple:
>> - a is implicitly converted to int using zext
>> - int operator~(int) has the unexpected behavior described below
>> - then icmp ne 0 is used to convert to bool and is always true
>> 
>> Here is the truth table of bool-to-int + operator~(int) + int-to-bool:
>> false → 0b0000 → 0b0000 → true
>> true → 0b0001 → 0b1110 → true
>> 
>> Note that we can solve the issue by using sext for bool-to-int conversions.
>> Should we? Is this a known problem?
>> 
>> -- 
>> Alexandre Isoard
>> 
>> 
>> -- 
>> Alexandre Isoard
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> 

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