[IRCE] Fix SCEVExpander's usage in IRCE

[mkazantsev]

Summary

When expanding exit conditions for pre- and postloops, we may end up expanding a
recurrency from the loop to in its loop's preheader. This produces incorrect IR.

This patch ensures that IRCE uses SCEVExpander correctly and only expands code which
is safe to expand in this particular location.

[mkazantsev]

Planning to add some more tests.

[mkazantsev]

Fixed typos in debug output message, added more tests for division.

[mkazantsev]

Ping

[reames]

LGTM once dependent changes land.

[mkazantsev]

Merged as https://reviews.llvm.org/rL318381

lib/Transforms/Scalar/InductiveRangeCheckElimination.cpp

else {
         DEBUG(dbgs() << "irce: could not prove no-overflow when computing "
                      << "preloop exit limit.  HighLimit = " << *(*SR.HighLimit)
                      << "\n");
         return false;
       }
       ExitPreLoopAtSCEV = SE.getAddExpr(*SR.HighLimit, MinusOneS);
     }
 
+    if (!isSafeToExpandAt(ExitPreLoopAtSCEV, InsertPt, SE)) {
+      DEBUG(dbgs() << "irce: could not prove that it is save to expand the"
+                   << " preloop exit limit " << *ExitPreLoopAtSCEV
+                   << " can be safely expanded at bloc; "
+                   << InsertPt->getParent()->getName() << "\n");
+      return false;
+    }
+
     ExitPreLoopAt = Expander.expandCodeFor(ExitPreLoopAtSCEV, IVTy, InsertPt);
     ExitPreLoopAt->setName("exit.preloop.at");
   }
 
   if (NeedsPostLoop) {
     const SCEV *ExitMainLoopAtSCEV = nullptr;
 
     if (Increasing)
       ExitMainLoopAtSCEV = *SR.HighLimit;
     else {
       if (CanBeMin(SE, *SR.LowLimit, IsSignedPredicate)) {
         DEBUG(dbgs() << "irce: could not prove no-overflow when computing "
                      << "mainloop exit limit.  LowLimit = " << *(*SR.LowLimit)
                      << "\n");
         return false;
       }
       ExitMainLoopAtSCEV = SE.getAddExpr(*SR.LowLimit, MinusOneS);
     }
 
+    if (!isSafeToExpandAt(ExitMainLoopAtSCEV, InsertPt, SE)) {
+      DEBUG(dbgs() << "irce: could not prove that it is save to expand the"
+                   << " main loop exit limit " << *ExitMainLoopAtSCEV
+                   << " can be safely expanded at block "
+                   << InsertPt->getParent()->getName() << "\n");
+      return false;
+    }
+
     ExitMainLoopAt = Expander.expandCodeFor(ExitMainLoopAtSCEV, IVTy, InsertPt);
     ExitMainLoopAt->setName("exit.mainloop.at");
   }
 
   // We clone these ahead of time so that we don't have to deal with changing
   // and temporarily invalid IR as we transform the loops.
   if (NeedsPreLoop)
     cloneLoop(PreLoop, "preloop");

test/Transforms/IRCE/bad_expander.ll

+; RUN: opt -verify-loop-info -irce-print-changed-loops -irce -S < %s 2>&1 | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128-ni:1"
+target triple = "x86_64-unknown-linux-gnu"
+
+; IRCE should fail here because the preheader's exiting value is a phi from the
+; loop, and this value cannot be expanded at loop's preheader.
+
+; Function Attrs: uwtable
+define void @test_01() {
+
+; CHECK-NOT:   irce: in function test_01: constrained Loop
+
+; CHECK-LABEL: test_01
+; CHECK-NOT:   preloop
+; CHECK-NOT:   postloop
+; CHECK-NOT:   br i1 false
+; CHECK-NOT:   br i1 true
+
+entry:
+  br label %loop
+
+exit:                                       ; preds = %guarded, %loop
+  ret void
+
+loop:                                      ; preds = %guarded, %entry
+  %indvars.iv = phi i64 [ 380, %entry ], [ %indvars.iv.next, %guarded ]
+  %tmp1 = phi i32 [ 3, %entry ], [ %tmp2, %guarded ]
+  %tmp2 = add nuw nsw i32 %tmp1, 1
+  %tmp3 = add nuw nsw i64 %indvars.iv, 1
+  %tmp4 = icmp slt i64 %tmp3, 5
+  br i1 %tmp4, label %guarded, label %exit
+
+guarded:                                          ; preds = %tmp0
+  %indvars.iv.next = add nsw i64 %indvars.iv, -1
+  %tmp5 = add nuw nsw i32 %tmp1, 8
+  %tmp6 = zext i32 %tmp5 to i64
+  %tmp7 = icmp eq i64 %indvars.iv.next, %tmp6
+  br i1 %tmp7, label %exit, label %loop
+}