[llvm-dev] Comparing Clang and GCC: only clang stores updated value in each iteration.

[Friedman, Eli via llvm-dev]

On 9/20/2018 6:52 AM, Jonas Paulsson via llvm-dev wrote:
> Hi,
>
> I have a benchmark (mcf) that is currently slower when compiled with 
> clang compared to gcc 8 (~10%). It seems that a hot loop has a few 
> differences, where one interesting one is that while clang stores an 
> incremented value in each iteration, gcc waits and just stores the 
> final value just once after the loop. The value is a global variable.
>
> I wonder if this is something clang does not do per default but can be 
> activated, similarly to the fp-contract situation?
>
> If not, is this a deficiency in clang? What pass should handle this? 
> IndVarSimplify?

See http://lists.llvm.org/pipermail/llvm-dev/2018-September/126064.html .

>
> I have made a reduced test case which shows the same difference 
> between the compilers: clang adds 1 and stores it back to 'a' in each 
> iteration, while gcc instead figures out the value a has after the 
> loop (0) and stores it then once to 'a'.

Your testcase is a bit weird because the condition of the while loop is 
the same as the condition of the if statement.  Is that really what the 
original loop looks like?

-Eli

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