[IndVars, RLEV] Support rewriting exit values in loops without known exits (prep work)

This is a prepatory patch for future work on support exit value rewriting in loops with a mixture of computable and non-computable exit counts.  The intention is to be "mostly NFC" - i.e. not enable any interesting new transforms - but in practice, there are some small output changes.

The test differences are caused by cases wherewhere getSCEVAtScope can simplify a single entry phi without needing any knowledge of the loop.

llvm-svn: 367485

Author: preames

llvm/lib/Transforms/Scalar/IndVarSimplify.cpp

@@ -630,7 +630,8 @@ bool IndVarSimplify::rewriteLoopExitValues(Loop *L, SCEVExpander &Rewriter) {
         // and varies predictably *inside* the loop.  Evaluate the value it
         // contains when the loop exits, if possible.
         const SCEV *ExitValue = SE->getSCEVAtScope(Inst, L->getParentLoop());
-        if (!SE->isLoopInvariant(ExitValue, L) ||
+        if (isa<SCEVCouldNotCompute>(ExitValue) ||
+            !SE->isLoopInvariant(ExitValue, L) ||
             !isSafeToExpand(ExitValue, *SE))
           continue;
 
@@ -804,7 +805,7 @@ bool IndVarSimplify::canLoopBeDeleted(
   L->getExitingBlocks(ExitingBlocks);
   SmallVector<BasicBlock *, 8> ExitBlocks;
   L->getUniqueExitBlocks(ExitBlocks);
-  if (ExitBlocks.size() > 1 || ExitingBlocks.size() > 1)
+  if (ExitBlocks.size() != 1 || ExitingBlocks.size() != 1)
     return false;
 
   BasicBlock *ExitBlock = ExitBlocks[0];
@@ -2774,14 +2775,11 @@ bool IndVarSimplify::run(Loop *L) {
   Rewriter.disableCanonicalMode();
   Changed |= simplifyAndExtend(L, Rewriter, LI);
 
-  // Check to see if this loop has a computable loop-invariant execution count.
-  // If so, this means that we can compute the final value of any expressions
+  // Check to see if we can compute the final value of any expressions
   // that are recurrent in the loop, and substitute the exit values from the
-  // loop into any instructions outside of the loop that use the final values of
-  // the current expressions.
-  //
-  if (ReplaceExitValue != NeverRepl &&
-      !isa<SCEVCouldNotCompute>(BackedgeTakenCount))
+  // loop into any instructions outside of the loop that use the final values
+  // of the current expressions.
+  if (ReplaceExitValue != NeverRepl)
     Changed |= rewriteLoopExitValues(L, Rewriter);
 
   // Eliminate redundant IV cycles.

llvm/test/Transforms/IndVarSimplify/lftr-pr20680.ll

@@ -62,7 +62,6 @@ define void @f() {
 ; CHECK:       cond.false.us:
 ; CHECK-NEXT:    br label [[COND_END_US]]
 ; CHECK:       cond.end.us:
-; CHECK-NEXT:    [[COND_US:%.*]] = phi i32 [ [[DIV]], [[COND_FALSE_US]] ], [ [[INDVARS_IV]], [[FOR_BODY3_US]] ]
 ; CHECK-NEXT:    [[TMP6:%.*]] = load i32, i32* @b, align 4
 ; CHECK-NEXT:    [[CMP91_US:%.*]] = icmp slt i32 [[TMP6]], 1
 ; CHECK-NEXT:    br i1 [[CMP91_US]], label [[FOR_INC_LR_PH_US:%.*]], label [[FOR_COND2_LOOPEXIT_US:%.*]]
@@ -79,10 +78,9 @@ define void @f() {
 ; CHECK-NEXT:    store i32 1, i32* @b, align 4
 ; CHECK-NEXT:    br label [[FOR_COND2_LOOPEXIT_US]]
 ; CHECK:       for.cond2.for.inc13_crit_edge.us-lcssa.us.us-lcssa:
-; CHECK-NEXT:    [[COND_LCSSA_PH_US_PH:%.*]] = phi i32 [ [[COND_US]], [[FOR_COND2_LOOPEXIT_US]] ]
 ; CHECK-NEXT:    br label [[FOR_COND2_FOR_INC13_CRIT_EDGE_US_LCSSA_US]]
 ; CHECK:       for.cond2.for.inc13_crit_edge.us-lcssa.us:
-; CHECK-NEXT:    [[COND_LCSSA_PH_US:%.*]] = phi i32 [ [[COND_LCSSA_PH_US_PH]], [[FOR_COND2_FOR_INC13_CRIT_EDGE_US_LCSSA_US_US_LCSSA]] ], [ [[DIV]], [[FOR_COND2_FOR_INC13_CRIT_EDGE_US_LCSSA_US_US_LCSSA_US]] ]
+; CHECK-NEXT:    [[COND_LCSSA_PH_US:%.*]] = phi i32 [ [[DIV]], [[FOR_COND2_FOR_INC13_CRIT_EDGE_US_LCSSA_US_US_LCSSA]] ], [ [[DIV]], [[FOR_COND2_FOR_INC13_CRIT_EDGE_US_LCSSA_US_US_LCSSA_US]] ]
 ; CHECK-NEXT:    br label [[FOR_COND2_FOR_INC13_CRIT_EDGE:%.*]]
 ; CHECK:       for.body3.lr.ph.split:
 ; CHECK-NEXT:    br i1 [[TOBOOL]], label [[FOR_BODY3_LR_PH_SPLIT_SPLIT_US:%.*]], label [[FOR_BODY3_LR_PH_SPLIT_FOR_BODY3_LR_PH_SPLIT_SPLIT_CRIT_EDGE:%.*]]

llvm/test/Transforms/IndVarSimplify/pr38674.ll

@@ -92,15 +92,13 @@ define i32 @test_02(i32 %x) {
 ; CHECK:       for.end:
 ; CHECK-NEXT:    br i1 false, label [[FOR_COND4_PREHEADER]], label [[FOR_END9:%.*]]
 ; CHECK:       for.end9:
-; CHECK-NEXT:    [[INC11:%.*]] = add nuw nsw i32 0, [[X]]
 ; CHECK-NEXT:    br i1 false, label [[FOR_COND1_PREHEADER]], label [[RETURN_LOOPEXIT3:%.*]]
 ; CHECK:       return.loopexit:
 ; CHECK-NEXT:    unreachable
 ; CHECK:       return.loopexit3:
-; CHECK-NEXT:    [[INC11_LCSSA:%.*]] = phi i32 [ [[INC11]], [[FOR_END9]] ]
 ; CHECK-NEXT:    br label [[RETURN:%.*]]
 ; CHECK:       return:
-; CHECK-NEXT:    ret i32 [[INC11_LCSSA]]
+; CHECK-NEXT:    ret i32 [[X]]
 ;
 entry:
   br label %for.cond1.preheader