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

David Blaikie via cfe-users cfe-users at lists.llvm.org
Wed Sep 25 12:57:20 PDT 2019

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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