[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



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@375379 91177308-0d34-0410-b5e6-96231b3b80d8

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()

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

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]]