[PATCH] Adding checker to detect excess padding in records

[PATCH] Adding checker to detect excess padding in records

The intent of this checker is to generate a report for any class / structure
that could reduce its padding by reordering the fields.  This results in a very
noisy checker.  To reduce the noise, this checker will currently only warn when
the number of bytes over "optimal" is more than 24.  This value is configurable
with -analyzer-config performance.Padding:AllowedPad=N.  Small values of
AllowedPad have the potential to generate hundreds of reports, and gigabytes
of HTML reports.

The checker searches for padding violations in two main ways.  First, it goes
record by record.  A report is generated if the fields could be reordered in a
way that reduces the padding by more than AllowedPad bytes.  Second, the
checker will generate a report if an array will cause more than AllowedPad
padding bytes to be generated.

The record checker currently skips many ABI specific cases.  Classes with base
classes are skipped because base class tail padding is ABI specific.  Bitfields
are just plain hard, and duplicating that code seems like a bad idea.  VLAs are
both uncommon and non-trivial to fix.

The array checker isn't very thorough right now.  It only checks to see if the
element type's fields could be reordered, and it doesn't recursively check to
see if any of the fields' fields could be reordered.  At some point in the
future, it would be nice if "arrays" could also look at array new usages and
malloc patterns that appear to be creating arrays.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@255545 91177308-0d34-0410-b5e6-96231b3b80d8