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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