[llvm-dev] A question about the widely used "xxx | true" inside llvm

[Craig Topper via llvm-dev]

The language requires both sides of the | need to be evaluated. The
compiler can't remove the function call if it has side effects.


~Craig


On Sun, Aug 19, 2018 at 10:15 PM Shoaib Meenai via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> This is better suited to llvm-dev, so I'm directing it there.
>
>
>
> *From: *llvm-commits <llvm-commits-bounces at lists.llvm.org> on behalf of
> Qing Shan Zhang via llvm-commits <llvm-commits at lists.llvm.org>
> *Reply-To: *Qing Shan Zhang <qshanz at cn.ibm.com>
> *Date: *Sunday, August 19, 2018 at 10:10 PM
> *To: *"llvm-commits at lists.llvm.org" <llvm-commits at lists.llvm.org>
> *Subject: *A question about the widely used "xxx | true" inside llvm
>
>
>
> Hi, guys,
>
>
>
> I notice that, we are widely using the statement "xxx | true" in llvm,
> such as "return SimplifyCFG(BB) | true;".  I have no idea if there is any
> downside to use the logic operator "||" here. Instead, I indeed see the
> potential problem to use bitwise operator "|". Because the bitwise operator
> do not short-circuit and the language std didn't specify the evaluation
> order for its lhs and rhs expression. That means, compiler is free to
> optimize this statement to "return true".
>
>
>
> In shorts, IMO, it is legal to do the following optimization:
> bool bar() {
>   return foo() | true;
> }
> -->
> bool bar() {
>   return true;  // the call to foo() is missing.
>
> }
>
>
> Best regards
>
> steven.zhang(张青山)
> XLC++ Compiler Frontend Developer
>
> IBM China Development Lab, Shanghai
> Tel: (8621)609-28454 Mobile: +8615900986116
> E-mail: qshanz at cn.ibm.com
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