Last week, ARMConstantIslandPass was failing to converge for the

MultiSource/Benchmarks/MiBench/automotive-susan test.  The failure has
since been masked by an unrelated change (just randomly), so I don't have
a testcase for this now.  Radar 7291928.

The situation where this happened is that a constant pool entry (CPE) was
placed at a lower address than the load that referenced it.  There were in
fact 2 CPEs placed at adjacent addresses and referenced by 2 loads that were
close together in the code.  The distance from the loads to the CPEs was
right at the limit of what they could handle, so that only one of the CPEs
could be placed within range.  On every iteration, the first CPE was found
to be out of range, causing a new CPE to be inserted.  The second CPE had
been in range but the newly inserted entry pushed it too far away.  Thus the
second CPE was also replaced by a new entry, which in turn pushed the first
CPE out of range.  Etc.

Judging from some comments in the code, the initial implementation of this
pass did not support CPEs placed _before_ their references.  In the case
where the CPE is placed at a higher address, the key to making the algorithm
terminate is that new CPEs are only inserted at the end of a group of adjacent
CPEs.  This is implemented by removing a basic block from the "WaterList"
once it has been used, and then adding the newly inserted CPE block to the
list so that the next insertion will come after it.  This avoids the ping-pong
effect where CPEs are repeatedly moved to the beginning of a group of
adjacent CPEs.  This does not work when going backwards, however, because the
entries at the end of an adjacent group of CPEs are closer than the CPEs
earlier in the group.

To make this pass terminate, we need to maintain a property that changes can
only happen in some sort of monotonic fashion.  The fix used here is to require
that the CPE for a particular constant pool load can only move to lower
addresses.  This is a very simple change to the code and should not cause
any significant degradation in the results.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@83902 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Target/ARM/ARMConstantIslandPass.cpp b/lib/Target/ARM/ARMConstantIslandPass.cpp
index e47ddd18a549f59fc0c0b94bfa8e0185f515f087..36eeffce2695b43ff055938fa4203e7e42fdb5c3 100644 (file)
--- a/lib/Target/ARM/ARMConstantIslandPass.cpp
+++ b/lib/Target/ARM/ARMConstantIslandPass.cpp
@@ -946,10 +946,11 @@ MachineBasicBlock *ARMConstantIslands::AcceptWater(water_iterator IP) {
 /// LookForWater - look for an existing entry in the WaterList in which
 /// we can place the CPE referenced from U so it's within range of U's MI.
 /// Returns true if found, false if not.  If it returns true, NewMBB
-/// is set to the WaterList entry.
-/// For ARM, we prefer the water that's farthest away. For Thumb, prefer
-/// water that will not introduce padding to water that will; within each
-/// group, prefer the water that's farthest away.
+/// is set to the WaterList entry.  For Thumb, prefer water that will not
+/// introduce padding to water that will.  To ensure that this pass
+/// terminates, the CPE location for a particular CPUser is only allowed to
+/// move to a lower address, so search backward from the end of the list and
+/// prefer the first water that is in range.
 bool ARMConstantIslands::LookForWater(CPUser &U, unsigned UserOffset,
                                       MachineBasicBlock *&NewMBB) {
   if (WaterList.empty())
@@ -960,7 +961,9 @@ bool ARMConstantIslands::LookForWater(CPUser &U, unsigned UserOffset,
   for (water_iterator IP = prior(WaterList.end()),
          B = WaterList.begin();; --IP) {
     MachineBasicBlock* WaterBB = *IP;
-    if (WaterIsInRange(UserOffset, WaterBB, U)) {
+    // Check if water is in range and at a lower address than the current one.
+    if (WaterIsInRange(UserOffset, WaterBB, U) &&
+        WaterBB->getNumber() < U.CPEMI->getParent()->getNumber()) {
       unsigned WBBId = WaterBB->getNumber();
       if (isThumb &&
           (BBOffsets[WBBId] + BBSizes[WBBId])%4 != 0) {
@@ -1109,10 +1112,7 @@ bool ARMConstantIslands::HandleConstantPoolUser(MachineFunction &MF,
   // We will be generating a new clone.  Get a UID for it.
   unsigned ID = AFI->createConstPoolEntryUId();
 
-  // Look for water where we can place this CPE.  We look for the farthest one
-  // away that will work.  Forward references only for now (although later
-  // we might find some that are backwards).
-
+  // Look for water where we can place this CPE.
   if (!LookForWater(U, UserOffset, NewMBB)) {
     // No water found.
     DEBUG(errs() << "No water found\n");