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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.<br>
<br>
--<br>
Chris<br>
<br>
<div class="moz-cite-prefix">On 25/09/2019 20:57, David Blaikie
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CAENS6Eu+iPedxYw8BNUNnUC7PvN7q5oUxXZRQwDM6AkJFZ7avQ@mail.gmail.com">
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<div dir="ltr">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).<br>
<br>
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.<br>
<br>
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.<br>
<br>
- Dave</div>
<br>
<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">On Wed, Sep 25, 2019 at 12:37
PM Christopher Williams via cfe-users <<a
href="mailto:cfe-users@lists.llvm.org"
moz-do-not-send="true">cfe-users@lists.llvm.org</a>>
wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Given
the code below, clang produces a constant value for test1,
test2<br>
and test4. Why doesn't it for test3?<br>
This is more of a curious query, than a request for help, but
if someone<br>
does have the answer, I'd appreciate as much detail as
possible.<br>
||<br>
<br>
|staticconstexprintcount_x(constchar*str){intcount{};for(;*str<br>
!=0;++str){count +=*str<br>
=='x';}returncount;}#defineSTRx1"123456789x"#defineSTRx4STRx1STRx1STRx1STRx1#defineSTRx8STRx4STRx4#defineSTRx16STRx8STRx8inttest1(){returncount_x(STRx4);}inttest2(){returncount_x(STRx8);}inttest3(){returncount_x(STRx16);}inttest4(){constexprautok<br>
=count_x(STRx16);returnk;}|<br>
<br>
<br>
|test1():# @test1()mov eax,4ret test2():# @test2()mov
eax,8ret<br>
test3():# @test3()xor eax,eax mov dl,49mov ecx,offset<br>
.L.str.2+1.LBB2_1:# =>This Inner Loop Header:
Depth=1xor esi,esi cmp<br>
dl,120sete sil add eax,esi movzx edx,byte ptr [rcx]add
rcx,1test<br>
dl,dl jne .LBB2_1 ret test4():# @test4()mov eax,16ret<br>
.L.str.2:.asciz<br>
"123456789x123456789x123456789x123456789x123456789x123456789x123456789x123456789x123456789x123456789x123456789x123456789x123456789x123456789x123456789x123456789x"|<br>
<br>
gcc does:<br>
<br>
|test1():mov eax,4ret test2():mov eax,8ret test3():mov
eax,16ret<br>
test4():mov eax,16ret|<br>
<br>
Compilation command lines used:<br>
<br>
|clang++-Ofast-std=c++2a-S -o --c src/test.cpp |grep
-Ev$'^\t+\\.'gcc9<br>
-Ofast-std=c++2a-S -o --c src/test.cpp |grep -Ev$'^\t+\\.'|<br>
<br>
Compiler Explorer: <a href="https://godbolt.org/z/V-3MEp"
rel="noreferrer" target="_blank" moz-do-not-send="true">https://godbolt.org/z/V-3MEp</a><br>
<br>
<br>
<br>
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</blockquote>
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