[Bug c++/98207] New: ARM64 IEEE math results change with -O2

[pramsey at cleverelephant dot ca]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98207

            Bug ID: 98207
           Summary: ARM64 IEEE math results change with -O2
           Product: gcc
           Version: 7.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pramsey at cleverelephant dot ca
  Target Milestone: ---

Created attachment 49707
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=49707&action=edit
Test program temp files built with O0

On the ARM64 platform (in particular testing on an AWS Graviton).

Linux ip-172-31-55-124.us-west-2.compute.internal
4.14.203-156.332.amzn2.aarch64 #1 SMP Fri Oct 30 19:19:46 UTC 2020 aarch64
aarch64 aarch64 GNU/Linux

A "small" program that carries out only normal arithmetic operations on double
returns a different results if g++ is called with -O0 (or with no O at all) and
-O2. 

Attached are dd-o0.ii and dd-o2.ii, the temp file outputs for compiling with
-O0 and -O2. 

On x84_64, g++ compiled code returns the same answer for both O0 and O2. 

On ARM64 and x84_64, clang++ returns the same answer for both O0 and O2. 

This test is extracted from a small routine that provides "double double"
precision math using only arithmetic operations on IEEE compliant doubles.
https://github.com/libgeos/geos/blob/master/src/math/DD.cpp