[ConstraintElim] Use cond from header as upper bound on IV in exit BB. (#94610)

For loops, we can use the condition in the loop header as upper bound on
the compared induction in the unique exit block, if it exists. This can
be done even if there are multiple in-loop edges to the unique exit
block, as any other exit may only exit earlier.

More generally, we could add the OR of all exit conditions to the exit,
but that's a possible future extension.

PR: #94610

Author: fhahn

llvm/lib/Transforms/Scalar/ConstraintElimination.cpp

@@ -1032,6 +1032,20 @@ void State::addInfoForInductions(BasicBlock &BB) {
   WorkList.push_back(FactOrCheck::getConditionFact(
       DTN, CmpInst::ICMP_SLT, PN, B,
       ConditionTy(CmpInst::ICMP_SLE, StartValue, B)));
+
+  // Try to add condition from header to the exit blocks. When exiting either
+  // with EQ or NE in the header, we know that the induction value must be u<=
+  // B, as other exits may only exit earlier.
+  assert(!StepOffset.isNegative() && "induction must be increasing");
+  assert((Pred == CmpInst::ICMP_EQ || Pred == CmpInst::ICMP_NE) &&
+         "unsupported predicate");
+  ConditionTy Precond = {CmpInst::ICMP_ULE, StartValue, B};
+  SmallVector<BasicBlock *> ExitBBs;
+  L->getExitBlocks(ExitBBs);
+  for (BasicBlock *EB : ExitBBs) {
+    WorkList.emplace_back(FactOrCheck::getConditionFact(
+        DT.getNode(EB), CmpInst::ICMP_ULE, A, B, Precond));
+  }
 }
 
 void State::addInfoFor(BasicBlock &BB) {

llvm/test/Transforms/ConstraintElimination/induction-condition-in-loop-exit.ll

@@ -19,8 +19,7 @@ define i1 @multi_exiting_loop_eq_same_unique_exit_const_compare_known(ptr %s) {
 ; CHECK-NEXT:    [[IV_NEXT]] = add nuw nsw i32 [[IV]], 1
 ; CHECK-NEXT:    br i1 [[LATCH_C]], label %[[LOOP_HEADER]], label %[[EXIT]]
 ; CHECK:       [[EXIT]]:
-; CHECK-NEXT:    [[T:%.*]] = icmp ult i32 [[IV]], 1235
-; CHECK-NEXT:    ret i1 [[T]]
+; CHECK-NEXT:    ret i1 true
 ;
 entry:
   br label %loop.header
@@ -99,8 +98,7 @@ define i1 @multi_exiting_loop_eq_same_unique_exit_const_compare_known_due_to_pre
 ; CHECK-NEXT:    [[IV_NEXT]] = add i32 [[IV]], 1
 ; CHECK-NEXT:    br i1 [[LATCH_C]], label %[[LOOP_HEADER]], label %[[EXIT]]
 ; CHECK:       [[EXIT]]:
-; CHECK-NEXT:    [[T:%.*]] = icmp ult i32 [[IV]], 1235
-; CHECK-NEXT:    ret i1 [[T]]
+; CHECK-NEXT:    ret i1 true
 ;
 entry:
   %pre.c = icmp ule i32 %start, 1234
@@ -341,8 +339,7 @@ define i1 @multi_exiting_loop_eq_same_unique_exit_var_compare_known(ptr %s, i32
 ; CHECK-NEXT:    [[IV_NEXT]] = add nuw nsw i32 [[IV]], 1
 ; CHECK-NEXT:    br i1 [[LATCH_C]], label %[[LOOP_HEADER]], label %[[EXIT]]
 ; CHECK:       [[EXIT]]:
-; CHECK-NEXT:    [[T:%.*]] = icmp ule i32 [[IV]], [[N]]
-; CHECK-NEXT:    ret i1 [[T]]
+; CHECK-NEXT:    ret i1 true
 ;
 entry:
   br label %loop.header
@@ -419,8 +416,7 @@ define i1 @multi_exiting_loop_ne_same_unique_exit_const_compare_known(ptr %s) {
 ; CHECK-NEXT:    [[IV_NEXT]] = add nuw nsw i32 [[IV]], 1
 ; CHECK-NEXT:    br i1 [[LATCH_C]], label %[[LOOP_HEADER]], label %[[EXIT]]
 ; CHECK:       [[EXIT]]:
-; CHECK-NEXT:    [[T:%.*]] = icmp ult i32 [[IV]], 1235
-; CHECK-NEXT:    ret i1 [[T]]
+; CHECK-NEXT:    ret i1 true
 ;
 entry:
   br label %loop.header
@@ -545,8 +541,7 @@ define i1 @multi_exiting_loop_eq_different_exits_2_const_compare_known(ptr %s, i
 ; CHECK-NEXT:    [[IV_NEXT]] = add nuw nsw i32 [[IV]], 1
 ; CHECK-NEXT:    br i1 [[LATCH_C]], label %[[LOOP_HEADER]], label %[[EXIT_2]]
 ; CHECK:       [[EXIT_1]]:
-; CHECK-NEXT:    [[T_1:%.*]] = icmp ult i32 [[IV]], 1235
-; CHECK-NEXT:    ret i1 [[T_1]]
+; CHECK-NEXT:    ret i1 true
 ; CHECK:       [[EXIT_2]]:
 ; CHECK-NEXT:    ret i1 true
 ;