[cfe-users] Why does clang not always produce constant value for same static constexpr

[David Blaikie via cfe-users]

On Wed, Sep 25, 2019 at 2:03 PM Christopher Williams <llvm2925-cw at ct1.xyz>
wrote:

> Thank you Dave. I have an understanding of constexpr evaluation, and
> realise the compiler is free to do what it likes in all but test4... I
> suppose I'd really like to know if there is an actual limit/threshold in
> place. If test3 is changed to use 100 characters it does as I expect, any
> more than that e.g. 101 and it bails. I also compiled with
> -Rpass-analysis='.*' -mllvm -print-after-all, and it *seems* the bail is in
> Induction Variable Simplify / Scalar Evolution, but I assume the actual
> problem could be before that.
>

Possible it's somewhere else, but without further evidence Induction
Variable Simplify/Scalar Evolution sounds like a perfectly plausible place
where such a threshold could be implemented so far as I know/could guess.


>
> --
> Chris
>
> On 25/09/2019 20:57, David Blaikie wrote:
>
> constexpr is a red herring here - except in 4, where you've used the
> constexpr keyword to create a constexpr context, in 1-3 these are just
> normal function calls the compiler optimizes as it sees fit - and it seems
> it saw fit to unroll and optimize to a constant cases 1 and 2, but not case
> 3 (perhaps because it was too long/some other middle-end optimization
> decided to bail out).
>
> I couldn't say for sure exactly which LLVM optimization bailed out early,
> or whether LLVM is using the same general approach as GCC here.
>
> Adding/removing the constexpr keyword from count_x shouldn't affect
> anything in cases 1-3 (in either Clang or GCC, really). But looks like it
> makes a big difference to GCC - perhaps GCC tries to evaluate constexpr in
> the frontend even when the language doesn't require it. Sounds like a
> recipe for some problematic compile-time to me... but don't know.
>
> - Dave
>
> On Wed, Sep 25, 2019 at 12:37 PM Christopher Williams via cfe-users <
> cfe-users at lists.llvm.org> wrote:
>
>> Given the code below, clang produces a constant value for test1, test2
>> and test4. Why doesn't it for test3?
>> This is more of a curious query, than a request for help, but if someone
>> does have the answer, I'd appreciate as much detail as possible.
>> ||
>>
>>     |staticconstexprintcount_x(constchar*str){intcount{};for(;*str
>>     !=0;++str){count +=*str
>>
>> =='x';}returncount;}#defineSTRx1"123456789x"#defineSTRx4STRx1STRx1STRx1STRx1#defineSTRx8STRx4STRx4#defineSTRx16STRx8STRx8inttest1(){returncount_x(STRx4);}inttest2(){returncount_x(STRx8);}inttest3(){returncount_x(STRx16);}inttest4(){constexprautok
>>     =count_x(STRx16);returnk;}|
>>
>>
>>     |test1():# @test1()mov eax,4ret test2():# @test2()mov eax,8ret
>>     test3():# @test3()xor eax,eax mov dl,49mov ecx,offset
>>     .L.str.2+1.LBB2_1:# =>This Inner Loop Header: Depth=1xor esi,esi cmp
>>     dl,120sete sil add eax,esi movzx edx,byte ptr [rcx]add rcx,1test
>>     dl,dl jne .LBB2_1 ret test4():# @test4()mov eax,16ret
>>     .L.str.2:.asciz
>>
>> "123456789x123456789x123456789x123456789x123456789x123456789x123456789x123456789x123456789x123456789x123456789x123456789x123456789x123456789x123456789x123456789x"|
>>
>> gcc does:
>>
>>     |test1():mov eax,4ret test2():mov eax,8ret test3():mov eax,16ret
>>     test4():mov eax,16ret|
>>
>> Compilation command lines used:
>>
>> |clang++-Ofast-std=c++2a-S -o --c src/test.cpp |grep -Ev$'^\t+\\.'gcc9
>> -Ofast-std=c++2a-S -o --c src/test.cpp |grep -Ev$'^\t+\\.'|
>>
>> Compiler Explorer: https://godbolt.org/z/V-3MEp
>>
>>
>>
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>
>
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