[llvm-commits] [llvm] r147926 - in /llvm/trunk: include/llvm/Analysis/ScalarEvolution.h lib/Analysis/ScalarEvolution.cpp

[Andrew Trick]

Author: atrick
Date: Wed Jan 11 00:52:55 2012
New Revision: 147926

URL: http://llvm.org/viewvc/llvm-project?rev=147926&view=rev
Log:
Clarified the SCEV getSmallConstantTripCount interface with in-your-face comments.

This interface is misleading and dangerous, but it is actually what we need for unrolling.

Modified:
    llvm/trunk/include/llvm/Analysis/ScalarEvolution.h
    llvm/trunk/lib/Analysis/ScalarEvolution.cpp

Modified: llvm/trunk/include/llvm/Analysis/ScalarEvolution.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Analysis/ScalarEvolution.h?rev=147926&r1=147925&r2=147926&view=diff
==============================================================================
--- llvm/trunk/include/llvm/Analysis/ScalarEvolution.h (original)
+++ llvm/trunk/include/llvm/Analysis/ScalarEvolution.h Wed Jan 11 00:52:55 2012
@@ -730,16 +730,21 @@
                                      const SCEV *LHS, const SCEV *RHS);
 
     /// getSmallConstantTripCount - Returns the maximum trip count of this loop
-    /// as a normal unsigned value, if possible. Returns 0 if the trip count is
-    /// unknown or not constant.
-    unsigned getSmallConstantTripCount(Loop *L, BasicBlock *ExitBlock);
+    /// as a normal unsigned value. Returns 0 if the trip count is unknown or
+    /// not constant. This "trip count" assumes that control exits via
+    /// ExitingBlock. More precisely, it is the number of times that control may
+    /// reach ExitingBlock before taking the branch. For loops with multiple
+    /// exits, it may not be the number times that the loop header executes if
+    /// the loop exits prematurely via another branch.
+    unsigned getSmallConstantTripCount(Loop *L, BasicBlock *ExitingBlock);
 
     /// getSmallConstantTripMultiple - Returns the largest constant divisor of
     /// the trip count of this loop as a normal unsigned value, if
     /// possible. This means that the actual trip count is always a multiple of
     /// the returned value (don't forget the trip count could very well be zero
-    /// as well!).
-    unsigned getSmallConstantTripMultiple(Loop *L, BasicBlock *ExitBlock);
+    /// as well!). As explained in the comments for getSmallConstantTripCount,
+    /// this assumes that control exits the loop via ExitingBlock.
+    unsigned getSmallConstantTripMultiple(Loop *L, BasicBlock *ExitingBlock);
 
     // getExitCount - Get the expression for the number of loop iterations for
     // which this loop is guaranteed not to exit via ExitingBlock. Otherwise

Modified: llvm/trunk/lib/Analysis/ScalarEvolution.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/ScalarEvolution.cpp?rev=147926&r1=147925&r2=147926&view=diff
==============================================================================
--- llvm/trunk/lib/Analysis/ScalarEvolution.cpp (original)
+++ llvm/trunk/lib/Analysis/ScalarEvolution.cpp Wed Jan 11 00:52:55 2012
@@ -3939,13 +3939,19 @@
 //
 
 /// getSmallConstantTripCount - Returns the maximum trip count of this loop as a
-/// normal unsigned value, if possible. Returns 0 if the trip count is unknown
-/// or not constant. Will also return 0 if the maximum trip count is very large
-/// (>= 2^32)
-unsigned ScalarEvolution::getSmallConstantTripCount(Loop *L,
-                                                    BasicBlock *ExitBlock) {
+/// normal unsigned value. Returns 0 if the trip count is unknown or not
+/// constant. Will also return 0 if the maximum trip count is very large (>=
+/// 2^32).
+///
+/// This "trip count" assumes that control exits via ExitingBlock. More
+/// precisely, it is the number of times that control may reach ExitingBlock
+/// before taking the branch. For loops with multiple exits, it may not be the
+/// number times that the loop header executes because the loop may exit
+/// prematurely via another branch.
+unsigned ScalarEvolution::
+getSmallConstantTripCount(Loop *L, BasicBlock *ExitingBlock) {
   const SCEVConstant *ExitCount =
-    dyn_cast<SCEVConstant>(getExitCount(L, ExitBlock));
+    dyn_cast<SCEVConstant>(getExitCount(L, ExitingBlock));
   if (!ExitCount)
     return 0;
 
@@ -3968,9 +3974,12 @@
 /// multiple of a constant (which is also the case if the trip count is simply
 /// constant, use getSmallConstantTripCount for that case), Will also return 1
 /// if the trip count is very large (>= 2^32).
-unsigned ScalarEvolution::getSmallConstantTripMultiple(Loop *L,
-                                                       BasicBlock *ExitBlock) {
-  const SCEV *ExitCount = getExitCount(L, ExitBlock);
+///
+/// As explained in the comments for getSmallConstantTripCount, this assumes
+/// that control exits the loop via ExitingBlock.
+unsigned ScalarEvolution::
+getSmallConstantTripMultiple(Loop *L, BasicBlock *ExitingBlock) {
+  const SCEV *ExitCount = getExitCount(L, ExitingBlock);
   if (ExitCount == getCouldNotCompute())
     return 1;