[PATCH] Add loop unroll pragma support

[Mark Heffernan]

Hi TylerNowicki, eliben, aaron.ballman, rsmith,

Piggy-backing on the support for "#pragma clang loop vectorize..." which was added recently by Tyler.  This patch adds support for loop unrolling pragmas.  The pragmas must immediately precede a loop statement and take the following forms:

#pragma clang loop unroll(enable)  // unroll the loop completely
#pragma clang loop unroll(disable)  // do not unroll the loop.
#pragma clang loop unroll_count(N)  // unroll the loop N times

if both unroll(enable) and unroll_count(N) are specified then the unroll_count takes precedence (ie, unroll the loop N times).

Tyler, I changed the logic a bit in CheckForIncompatibleAttributes, specifically it now rejects any combination of the disable form of the pragma and the numeric form (eg, vectorize(disable) and vectorize_width(N)).  As a special case, it previously allowed this combination if the numeric value is 1.  The logic seems cleaner without that special case.  Lemme know if that's reasonable.

I'll be sending out a LLVM patch which consumes the generated metadata right after this.

http://reviews.llvm.org/D4089

Files:
  include/clang/Basic/Attr.td
  include/clang/Basic/DiagnosticParseKinds.td
  lib/CodeGen/CGStmt.cpp
  lib/Parse/ParsePragma.cpp
  lib/Sema/SemaStmtAttr.cpp
  test/CodeGen/pragma-loop.cpp
  test/PCH/pragma-loop.cpp
  test/Parser/pragma-loop.cpp
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