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* [Bug c++/98207] New: ARM64 IEEE math results change with -O2 @ 2020-12-08 19:53 pramsey at cleverelephant dot ca 2020-12-08 19:54 ` [Bug c++/98207] " pramsey at cleverelephant dot ca ` (3 more replies) 0 siblings, 4 replies; 5+ messages in thread From: pramsey at cleverelephant dot ca @ 2020-12-08 19:53 UTC (permalink / raw) To: gcc-bugs 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 ^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c++/98207] ARM64 IEEE math results change with -O2 2020-12-08 19:53 [Bug c++/98207] New: ARM64 IEEE math results change with -O2 pramsey at cleverelephant dot ca @ 2020-12-08 19:54 ` pramsey at cleverelephant dot ca 2020-12-08 20:04 ` pinskia at gcc dot gnu.org ` (2 subsequent siblings) 3 siblings, 0 replies; 5+ messages in thread From: pramsey at cleverelephant dot ca @ 2020-12-08 19:54 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98207 --- Comment #1 from Paul Ramsey <pramsey at cleverelephant dot ca> --- Created attachment 49708 --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=49708&action=edit Test program temp files built with O2 ^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c++/98207] ARM64 IEEE math results change with -O2 2020-12-08 19:53 [Bug c++/98207] New: ARM64 IEEE math results change with -O2 pramsey at cleverelephant dot ca 2020-12-08 19:54 ` [Bug c++/98207] " pramsey at cleverelephant dot ca @ 2020-12-08 20:04 ` pinskia at gcc dot gnu.org 2020-12-08 20:05 ` pinskia at gcc dot gnu.org 2020-12-08 20:21 ` pramsey at cleverelephant dot ca 3 siblings, 0 replies; 5+ messages in thread From: pinskia at gcc dot gnu.org @ 2020-12-08 20:04 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98207 Andrew Pinski <pinskia at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- Status|UNCONFIRMED |RESOLVED Resolution|--- |INVALID --- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> --- The default is to use fused multiple add. If you don't want to use FMA, then use -ffp-contract=off (the default =fast). x84_64 by default does not have a fused-multiple-add instruction without extra options (to enable the ISA for fused multiple add instruction). ^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c++/98207] ARM64 IEEE math results change with -O2 2020-12-08 19:53 [Bug c++/98207] New: ARM64 IEEE math results change with -O2 pramsey at cleverelephant dot ca 2020-12-08 19:54 ` [Bug c++/98207] " pramsey at cleverelephant dot ca 2020-12-08 20:04 ` pinskia at gcc dot gnu.org @ 2020-12-08 20:05 ` pinskia at gcc dot gnu.org 2020-12-08 20:21 ` pramsey at cleverelephant dot ca 3 siblings, 0 replies; 5+ messages in thread From: pinskia at gcc dot gnu.org @ 2020-12-08 20:05 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98207 --- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> --- (In reply to Andrew Pinski from comment #2) > The default is to use fused multiple add. > If you don't want to use FMA, then use -ffp-contract=off (the default =fast). > > > x84_64 by default does not have a fused-multiple-add instruction without > extra options (to enable the ISA for fused multiple add instruction). Note this is still IEEE compliant FP and C standard compliant too. ^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c++/98207] ARM64 IEEE math results change with -O2 2020-12-08 19:53 [Bug c++/98207] New: ARM64 IEEE math results change with -O2 pramsey at cleverelephant dot ca ` (2 preceding siblings ...) 2020-12-08 20:05 ` pinskia at gcc dot gnu.org @ 2020-12-08 20:21 ` pramsey at cleverelephant dot ca 3 siblings, 0 replies; 5+ messages in thread From: pramsey at cleverelephant dot ca @ 2020-12-08 20:21 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98207 --- Comment #4 from Paul Ramsey <pramsey at cleverelephant dot ca> --- Thanks for the pointer! ^ permalink raw reply [flat|nested] 5+ messages in thread
end of thread, other threads:[~2020-12-08 20:21 UTC | newest] Thread overview: 5+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 2020-12-08 19:53 [Bug c++/98207] New: ARM64 IEEE math results change with -O2 pramsey at cleverelephant dot ca 2020-12-08 19:54 ` [Bug c++/98207] " pramsey at cleverelephant dot ca 2020-12-08 20:04 ` pinskia at gcc dot gnu.org 2020-12-08 20:05 ` pinskia at gcc dot gnu.org 2020-12-08 20:21 ` pramsey at cleverelephant dot ca
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