Bug 1388 – LTE module doesn't build in optimized mode with GCC 4.7

Bug 1388 - LTE module doesn't build in optimized mode with GCC 4.7


Summary:	LTE module doesn't build in optimized mode with GCC 4.7

Status:	RESOLVED FIXED

Product:	ns-3
Classification:	Unclassified
Component:	lte
Version:	pre-release
Hardware:	All Linux

Importance:	P5 normal
Assigned To:	Giuseppe Piro

URL:
Keywords:

Depends on:
Blocks:
	Show dependency tree / graph

Reported:	2012-03-08 18:59 UTC by Vedran Miletić
Modified:	2012-03-18 15:45 UTC (History)
CC List:	1 user (show)

See Also:

Attachments
Introduce break in while loop (526 bytes, patch) 2012-03-08 18:59 UTC, Vedran Miletić	Details \| Diff
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Description Vedran Miletić 2012-03-08 18:59:35 UTC

Created attachment 1352 [details]
Introduce break in while loop

[ 738/1225] cxx: src/lte/model/amc-module.cc -> build/optimized/src/lte/model/amc-module.cc.1.o
../../src/lte/model/amc-module.cc: In member function 'int ns3::AmcModule::GetCqiFromSpectralEfficiency(double)':
../../src/lte/model/amc-module.cc:123:43: error: array subscript is above array bounds [-Werror=array-bounds]
cc1plus: all warnings being treated as errors

To the best of my understanding, this patch produces functionality wise eqivalent code that builds with GCC 4.7. All tests pass.

Comment 1 Vedran Miletić 2012-03-15 08:33:16 UTC

Giuseppe, is this code OK for inclusion?

Comment 2 Giuseppe Piro 2012-03-15 10:35:59 UTC

The patch is OK for me.

Comment 3 Tommaso Pecorella 2012-03-15 18:13:15 UTC

I'm not happy with the patch... sorry if I'm pedantic.

The problem seems to be more subtle, and silencing the compiler by obfuscating the code isn't very wise. I have to confess that the compiler is smart indeed, and it found an obscure vector boundary issue.

  while (SpectralEfficiencyForCqiIndex[cqi] < s && cqi <= 14)
  { [...] }
Problem: the check is done sequentially (it's an and), so when cqi is 15 the first vector will go out of boundaries. The second check will return false and the while will stop.

A smarter solution is to invert the checks.
  while ( cqi <= 14 && SpectralEfficiencyForCqiIndex[cqi] < s )
  { [...] }

This should make the compiler happy again. I can't check, but if the error goes away, this is way cleaner.

Cheers,

T.

Comment 4 Vedran Miletić 2012-03-16 12:39:09 UTC

Tommaso, you are right, just tested it and it works. Talk about commutativity of && operation.

While at being pedantic about this while (pun intended), let's go a bit further and make it like:

  while (cqi < 15 && SpectralEfficiencyForCqiIndex[cqi] < s)

which is somewhat more C++ style.

Whichever way, let's commit this.

Comment 5 Tommaso Pecorella 2012-03-16 16:08:30 UTC

+1, go ahead.

T.

Comment 6 Vedran Miletić 2012-03-18 15:45:21 UTC

changeset:   7768:7d43ccc63a96
tag:         tip
user:        Vedran Miletić <rivanvx@gmail.com>
date:        Sun Mar 18 20:43:18 2012 +0100
summary:     Bug 1388 - LTE module doesn't build in optimized mode with GCC 4.7