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

[Christopher Williams via cfe-users]

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.

--
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 <mailto: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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