________________________________________
From: Vishal Subramanyam <vishalsubramanyam@xxxxxxxxxxx>
Sent: Sunday, January 23, 2022 9:56 AM
To: Xi Ruoyao
Subject: Re: Pure/const function not getting executed as the first operand to logical OR ( || ) (C++)
This still doesn't explain why an O1 level optimization can violate the standard by not evaluating the first operand.
What optimization is my code triggering with -fno-inline? My function clearly has a return value, so how did the compiler
decide that the return value doesn't matter?
Thanks,
Vishal
________________________________________
From: Xi Ruoyao <xry111@xxxxxxxxxxxxxxxx>
Sent: Sunday, January 23, 2022 9:12 AM
To: Vishal Subramanyam; gcc-help@xxxxxxxxxxx
Subject: Re: Pure/const function not getting executed as the first operand to logical OR ( || ) (C++)
On Sun, 2022-01-23 at 08:37 +0000, Vishal Subramanyam via Gcc-help
wrote:
> When compiling the following C++ snippet with -O0, I'm getting a
> Floating Point Exception on my Ubuntu 21.10, g++ 11.2.0, x86-64.
>
> #include <iostream>
>
> int func(int n, int r)
> {
> return n % r;
> }
> int main()
> {
> const int n = 15, r = 0;
> if (r == 0 || func(n, r))
> std::cout << "YES" << std::endl;
> else
> std::cout << "NO" << std::endl;
> if (func(n, r) || r == 0)
> std::cout << "YES" << std::endl;
> else
> std::cout << "NO" << std::endl;
> return 0;
> }
>
> With -O0, I'm getting a Floating Point error in the second if
> condition since it involves executing the "func" function where
> division by zero occurs.
>
> YES
> Floating point exception (core dumped)
>
> Now, with -O1 optimization, this is the output.
>
> YES
> YES
>
> The "func" call has been optimized away. I checked the assembly code
> output with -S and verified the same. Now, I would like to know what
> was the optimization (the optimization option) that led to this
> happening. When I tried compiling the same code with -O0 but with an
> additional "-fipa-pure-const", I'm getting the same output as the O1
> program with two YESes.
I can reproduce this behavior, but I have some doubt on this: I remember
someone told me that -f* options had no effect with -O0.
> But when I try compiling with both the -O1 and -fno-ipa-pure-const, I
> am not getting the Floating Point Exception. If -fipa-pure-const is
> responsible for the optimization that removes the function call,
> shouldn't the function call occur with -fno-ipa-pure-const. I have
> tried various combinations of options related to dead code elimination
> and branch probabilities, but can't figure out what's happening?
You need -fno-inline. If the function call is inlined completely, then
the compiler can find and remove dead code without any inter-procedural
analysis.
> g++ t.cc -O1 -fno-inline -fno-ipa-pure-const && ./a.out
> YES
> [1] 4151 floating point exception (core dumped) ./a.out
--
Xi Ruoyao <xry111@xxxxxxxxxxxxxxxx>
School of Aerospace Science and Technology, Xidian University
