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ICPC missing an optimisation opportunity?

Martin_K_6 — Thu, 12 Sep 2013 07:19:41 GMT

Using either icpc 13.1.1 or 14.0.0 and I'm observing the same optimisation issue. A statement of the form:

[cpp]A a = A().mutator();[/cpp]

where A has an empty default ctor, could causes the compiler to route the result from the mutator via the temporary, rather than route directly to the lvalue. The drawback in the former case is that there is a superfluous copy to the lvalue.

I've attached a code sample containing two exhibits. In Exibit A the compiler eliminates the temporary; in Exibit B the compiler does not eliminate the temporary.

Compilers tested:

[bash]$ icc -V Intel(R) C Intel(R) 64 Compiler XE for applications running on Intel(R) 64, Version 13.1.1.163 Build 20130313 Copyright (C) 1985-2013 Intel Corporation. All rights reserved. .. $ icc -V Intel(R) C Intel(R) 64 Compiler XE for applications running on Intel(R) 64, Version 14.0.0.080 Build 20130728 Copyright (C) 1985-2013 Intel Corporation. All rights reserved.[/bash]

CLI options used: -pipe -fno-exceptions -fno-rtti -fstrict-aliasing -unroll-aggressive -mmic -funroll-loops -O3 -DNDEBUG

Added attachment missing from

Martin_K_6 — Thu, 12 Sep 2013 07:21:43 GMT

Added attachment missing from OP.

Thank you for reporting this

Kevin_D_Intel — Thu, 12 Sep 2013 11:21:00 GMT

Thank you for reporting this possible missed optimization opportunity and for the convenient test case. I routed it to Development (see internal tracking id below) and will let you know what I hear.

(Internal tracking id: DPD200247989)
(Resolution Update on 09/08/2014): This defect is fixed in the Intel® Parallel Studio XE 2015 Initial Release (2015.0.090 - Linux)

Thank you for the quick reply

Martin_K_6 — Thu, 12 Sep 2013 18:32:57 GMT

Thank you for the quick reply, Kevin!

The compiler changes needed

Kevin_D_Intel — Mon, 08 Sep 2014 17:09:41 GMT

The compiler changes needed to eliminate the temporary used in Exhibit B in the original test case are now available in the new Intel Parallel Studio XE 2015 release for Linux (Version 15.0.0.090 Build 20140723) now available from the Intel Registration Center.