[IndVars] Eliminate loop exits with equivalent exit counts

We can end up with two loop exits whose exit counts are equivalent, but whose textual representation is different and non-obvious. For the sub-case where we have a series of exits which dominate one another (common), eliminate any exits which would iterate *after* a previous exit on the exiting iteration.

As noted in the TODO being removed, I'd always thought this was a good idea, but I've now seen this in a real workload as well.

Interestingly, in review, Nikita pointed out there's let another oppurtunity to leverage SCEV's reasoning.  If we kept track of the min of dominanting exits so far, we could discharge exits with EC >= MDE.  This is less powerful than the existing transform (since later exits aren't considered), but potentially more powerful for any case where SCEV can prove a >= b, but neither a == b or a > b.  I don't have an example to illustrate that oppurtunity, but won't be suprised if we find one and return to handle that case as well.  

Differential Revision: https://reviews.llvm.org/D69009

llvm-svn: 375379

llvm/lib/Transforms/Scalar/IndVarSimplify.cpp

@@ -2717,6 +2717,24 @@ bool IndVarSimplify::optimizeLoopExits(Loop *L, SCEVExpander &Rewriter) {
   if (isa<SCEVCouldNotCompute>(MaxExitCount))
     return false;
 
+  // Visit our exit blocks in order of dominance.  We know from the fact that
+  // all exits (left) are analyzeable that the must be a total dominance order
+  // between them as each must dominate the latch.  The visit order only
+  // matters for the provably equal case.  
+  llvm::sort(ExitingBlocks,
+             [&](BasicBlock *A, BasicBlock *B) {
+               // std::sort sorts in ascending order, so we want the inverse of
+               // the normal dominance relation.
+               if (DT->properlyDominates(A, B)) return true;
+               if (DT->properlyDominates(B, A)) return false;
+               llvm_unreachable("expected total dominance order!");
+             });
+#ifdef ASSERT
+  for (unsigned i = 1; i < ExitingBlocks.size(); i++) {
+    assert(DT->dominates(ExitingBlocks[i-1], ExitingBlocks[i]));
+  }
+#endif
+  
   auto FoldExit = [&](BasicBlock *ExitingBB, bool IsTaken) {
     BranchInst *BI = cast<BranchInst>(ExitingBB->getTerminator());
     bool ExitIfTrue = !L->contains(*succ_begin(ExitingBB));
@@ -2729,6 +2747,7 @@ bool IndVarSimplify::optimizeLoopExits(Loop *L, SCEVExpander &Rewriter) {
   };
 
   bool Changed = false;
+  SmallSet<const SCEV*, 8> DominatingExitCounts;
   for (BasicBlock *ExitingBB : ExitingBlocks) {
     const SCEV *ExitCount = SE->getExitCount(L, ExitingBB);
     assert(!isa<SCEVCouldNotCompute>(ExitCount) && "checked above");
@@ -2766,10 +2785,15 @@ bool IndVarSimplify::optimizeLoopExits(Loop *L, SCEVExpander &Rewriter) {
       continue;
     }
 
-    // TODO: If we can prove that the exiting iteration is equal to the exit
-    // count for this exit and that no previous exit oppurtunities exist within
-    // the loop, then we can discharge all other exits.  (May fall out of
-    // previous TODO.)
+    // As we run, keep track of which exit counts we've encountered.  If we
+    // find a duplicate, we've found an exit which would have exited on the
+    // exiting iteration, but (from the visit order) strictly follows another
+    // which does the same and is thus dead. 
+    if (!DominatingExitCounts.insert(ExitCount).second) {
+      FoldExit(ExitingBB, false);
+      Changed = true;
+      continue;
+    }
   }
   return Changed;
 }

llvm/test/Transforms/IndVarSimplify/eliminate-exit.ll

@@ -185,5 +185,39 @@ exit:
   ret void
 }
 
+define void @mixed_width(i32 %len) {
+; CHECK-LABEL: @mixed_width(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[LEN_ZEXT:%.*]] = zext i32 [[LEN:%.*]] to i64
+; CHECK-NEXT:    br label [[LOOP:%.*]]
+; CHECK:       loop:
+; CHECK-NEXT:    [[IV:%.*]] = phi i64 [ 0, [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[BACKEDGE:%.*]] ]
+; CHECK-NEXT:    [[IV_NEXT]] = add nuw nsw i64 [[IV]], 1
+; CHECK-NEXT:    [[CMP1:%.*]] = icmp ult i64 [[IV]], [[LEN_ZEXT]]
+; CHECK-NEXT:    br i1 [[CMP1]], label [[BACKEDGE]], label [[EXIT:%.*]]
+; CHECK:       backedge:
+; CHECK-NEXT:    call void @side_effect()
+; CHECK-NEXT:    br i1 true, label [[LOOP]], label [[EXIT]]
+; CHECK:       exit:
+; CHECK-NEXT:    ret void
+;
+entry:
+  %len.zext = zext i32 %len to i64
+  br label %loop
+loop:
+  %iv = phi i64 [0, %entry], [%iv.next, %backedge]
+  %iv2 = phi i32 [0, %entry], [%iv2.next, %backedge]
+  %iv.next = add i64 %iv, 1
+  %iv2.next = add i32 %iv2, 1
+  %cmp1 = icmp ult i64 %iv, %len.zext
+  br i1 %cmp1, label %backedge, label %exit
+
+backedge:
+  call void @side_effect()
+  %cmp2 = icmp ult i32 %iv2, %len
+  br i1 %cmp2, label %loop, label %exit
+exit:
+  ret void
+}
 
 declare void @side_effect()

llvm/test/Transforms/IndVarSimplify/loop-predication.ll

@@ -464,7 +464,6 @@ define i32 @duplicate_checks(i32* %array.1, i32* %array.2, i32* %array.3, i32 %l
 ; CHECK-NEXT:    [[TMP2:%.*]] = icmp ult i32 [[LENGTH:%.*]], [[TMP1]]
 ; CHECK-NEXT:    [[UMIN:%.*]] = select i1 [[TMP2]], i32 [[LENGTH]], i32 [[TMP1]]
 ; CHECK-NEXT:    [[TMP3:%.*]] = icmp ne i32 [[LENGTH]], [[UMIN]]
-; CHECK-NEXT:    [[TMP4:%.*]] = icmp ne i32 [[LENGTH]], [[UMIN]]
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
 ; CHECK-NEXT:    [[LOOP_ACC:%.*]] = phi i32 [ [[LOOP_ACC_NEXT:%.*]], [[GUARDED1:%.*]] ], [ 0, [[LOOP_PREHEADER:%.*]] ]
@@ -478,7 +477,7 @@ define i32 @duplicate_checks(i32* %array.1, i32* %array.2, i32* %array.3, i32 %l
 ; CHECK-NEXT:    [[ARRAY_1_I_PTR:%.*]] = getelementptr inbounds i32, i32* [[ARRAY_1:%.*]], i64 [[I_I64]]
 ; CHECK-NEXT:    [[ARRAY_1_I:%.*]] = load i32, i32* [[ARRAY_1_I_PTR]], align 4
 ; CHECK-NEXT:    [[LOOP_ACC_1:%.*]] = add i32 [[LOOP_ACC]], [[ARRAY_1_I]]
-; CHECK-NEXT:    br i1 [[TMP4]], label [[GUARDED1]], label [[DEOPT2:%.*]], !prof !0
+; CHECK-NEXT:    br i1 true, label [[GUARDED1]], label [[DEOPT2:%.*]], !prof !0
 ; CHECK:       deopt2:
 ; CHECK-NEXT:    call void @prevent_merging()
 ; CHECK-NEXT:    ret i32 -1
@@ -784,7 +783,7 @@ exit:
 ; If we have a dominating exit (exit1) which can't be itself rewritten, we
 ; can't rewrite a later exit (exit2).  Doing so would cause the loop to exit
 ; from the exit2 when it should have exited from exit1.
-define i32 @neg_dominating_exit(i32* %array, i32 %length, i32 %n) {
+define i32 @neg_dominating_exit(i32* %array, i32 %length, i32 %length2, i32 %n) {
 ; CHECK-LABEL: @neg_dominating_exit(
 ; CHECK-NEXT:  loop.preheader:
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
@@ -798,7 +797,7 @@ define i32 @neg_dominating_exit(i32* %array, i32 %length, i32 %n) {
 ; CHECK-NEXT:    call void @prevent_merging()
 ; CHECK-NEXT:    ret i32 [[RESULT]]
 ; CHECK:       guarded:
-; CHECK-NEXT:    [[WITHIN_BOUNDS2:%.*]] = icmp ult i32 [[I]], [[LENGTH]]
+; CHECK-NEXT:    [[WITHIN_BOUNDS2:%.*]] = icmp ult i32 [[I]], [[LENGTH2:%.*]]
 ; CHECK-NEXT:    br i1 [[WITHIN_BOUNDS2]], label [[GUARDED2]], label [[DEOPT2:%.*]], !prof !0
 ; CHECK:       deopt2:
 ; CHECK-NEXT:    call void @prevent_merging()
@@ -830,7 +829,7 @@ deopt:                                            ; preds = %loop
   ret i32 %result
 
 guarded:                                          ; preds = %loop
-  %within.bounds2 = icmp ult i32 %i, %length
+  %within.bounds2 = icmp ult i32 %i, %length2
   br i1 %within.bounds2, label %guarded2, label %deopt2, !prof !0
 
 deopt2:                                            ; preds = %loop

llvm/test/Transforms/IndVarSimplify/pr38674.ll

@@ -14,10 +14,9 @@ define i32 @test_01() {
 ; CHECK-NEXT:    [[ZEXT:%.*]] = zext i16 1 to i32
 ; CHECK-NEXT:    br label [[FOR_BODY6:%.*]]
 ; CHECK:       for.cond4:
-; CHECK-NEXT:    [[CMP5:%.*]] = icmp ult i32 [[INC:%.*]], 2
-; CHECK-NEXT:    br i1 [[CMP5]], label [[FOR_BODY6]], label [[FOR_END:%.*]]
+; CHECK-NEXT:    br i1 true, label [[FOR_BODY6]], label [[FOR_END:%.*]]
 ; CHECK:       for.body6:
-; CHECK-NEXT:    [[IV:%.*]] = phi i32 [ 0, [[FOR_COND4_PREHEADER]] ], [ [[INC]], [[FOR_COND4:%.*]] ]
+; CHECK-NEXT:    [[IV:%.*]] = phi i32 [ 0, [[FOR_COND4_PREHEADER]] ], [ [[INC:%.*]], [[FOR_COND4:%.*]] ]
 ; CHECK-NEXT:    [[TMP0:%.*]] = icmp eq i32 [[IV]], [[ZEXT]]
 ; CHECK-NEXT:    [[INC]] = add nuw nsw i32 [[IV]], 1
 ; CHECK-NEXT:    br i1 [[TMP0]], label [[RETURN_LOOPEXIT:%.*]], label [[FOR_COND4]]