[llvm] r364135 - Exploit a zero LoopExit count to eliminate loop exits

[Philip Reames via llvm-commits]

Author: reames
Date: Sat Jun 22 10:54:25 2019
New Revision: 364135

URL: http://llvm.org/viewvc/llvm-project?rev=364135&view=rev
Log:
Exploit a zero LoopExit count to eliminate loop exits

This turned out to be surprisingly effective. I was originally doing this just for completeness sake, but it seems like there are a lot of cases where SCEV's exit count reasoning is stronger than it's isKnownPredicate reasoning.

Once this is in, I'm thinking about trying to build on the same infrastructure to eliminate provably untaken checks. There may be something generally interesting here.

Differential Revision: https://reviews.llvm.org/D63618


Modified:
    llvm/trunk/lib/Transforms/Scalar/IndVarSimplify.cpp
    llvm/trunk/test/Transforms/IndVarSimplify/eliminate-comparison.ll
    llvm/trunk/test/Transforms/IndVarSimplify/eliminate-trunc.ll

Modified: llvm/trunk/lib/Transforms/Scalar/IndVarSimplify.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/IndVarSimplify.cpp?rev=364135&r1=364134&r2=364135&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Scalar/IndVarSimplify.cpp (original)
+++ llvm/trunk/lib/Transforms/Scalar/IndVarSimplify.cpp Sat Jun 22 10:54:25 2019
@@ -2724,9 +2724,21 @@ bool IndVarSimplify::run(Loop *L) {
       if (isa<SCEVCouldNotCompute>(ExitCount))
         continue;
 
-      // Better to fold to true (TODO: do so!)
-      if (ExitCount->isZero())
+      // If we know we'd exit on the first iteration, rewrite the exit to
+      // reflect this.  This does not imply the loop must exit through this
+      // exit; there may be an earlier one taken on the first iteration.
+      // TODO: Given we know the backedge can't be taken, we should go ahead
+      // and break it.  Or at least, kill all the header phis and simplify.
+      if (ExitCount->isZero()) {
+        auto *BI = cast<BranchInst>(ExitingBB->getTerminator());
+        bool ExitIfTrue = !L->contains(*succ_begin(ExitingBB));
+        auto *NewCond = ExitIfTrue ?
+          ConstantInt::getTrue(BI->getCondition()->getType()) :
+          ConstantInt::getFalse(BI->getCondition()->getType());
+        BI->setCondition(NewCond);
+        Changed = true;
         continue;
+      }
       
       PHINode *IndVar = FindLoopCounter(L, ExitingBB, ExitCount, SE, DT);
       if (!IndVar)

Modified: llvm/trunk/test/Transforms/IndVarSimplify/eliminate-comparison.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/IndVarSimplify/eliminate-comparison.ll?rev=364135&r1=364134&r2=364135&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/IndVarSimplify/eliminate-comparison.ll (original)
+++ llvm/trunk/test/Transforms/IndVarSimplify/eliminate-comparison.ll Sat Jun 22 10:54:25 2019
@@ -284,7 +284,7 @@ define i32 @func_12() nounwind uwtable {
 ; CHECK-NEXT:    [[SEXT21:%.*]] = shl i32 [[TMP57]], 16
 ; CHECK-NEXT:    [[TMP76:%.*]] = icmp ne i32 [[SEXT34]], [[SEXT21]]
 ; CHECK-NEXT:    [[TMP81]] = add nuw nsw i32 [[__KEY8_0]], 1
-; CHECK-NEXT:    br i1 [[TMP76]], label [[FORCOND38]], label [[ASSERT77:%.*]]
+; CHECK-NEXT:    br i1 false, label [[FORCOND38]], label [[ASSERT77:%.*]]
 ; CHECK:       assert77:
 ; CHECK-NEXT:    tail call void @llvm.trap()
 ; CHECK-NEXT:    unreachable
@@ -897,5 +897,4 @@ leave:
   ret void
 }
 
-
 !0 = !{i32 0, i32 2147483647}

Modified: llvm/trunk/test/Transforms/IndVarSimplify/eliminate-trunc.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/IndVarSimplify/eliminate-trunc.ll?rev=364135&r1=364134&r2=364135&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/IndVarSimplify/eliminate-trunc.ll (original)
+++ llvm/trunk/test/Transforms/IndVarSimplify/eliminate-trunc.ll Sat Jun 22 10:54:25 2019
@@ -174,7 +174,7 @@ define void @test_06(i32 %n) {
 ; CHECK-NEXT:    [[IV_NEXT]] = add nuw nsw i64 [[IV]], 1
 ; CHECK-NEXT:    [[NARROW_IV:%.*]] = trunc i64 [[IV]] to i32
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i32 [[NARROW_IV]], [[N:%.*]]
-; CHECK-NEXT:    br i1 [[CMP]], label [[LOOP]], label [[EXIT:%.*]]
+; CHECK-NEXT:    br i1 false, label [[LOOP]], label [[EXIT:%.*]]
 ; CHECK:       exit:
 ; CHECK-NEXT:    ret void
 ;
@@ -354,7 +354,7 @@ define void @test_06_unsigned(i32 %n) {
 ; CHECK-NEXT:    [[IV_NEXT]] = add nsw i64 [[IV]], 1
 ; CHECK-NEXT:    [[NARROW_IV:%.*]] = trunc i64 [[IV]] to i32
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp ult i32 [[NARROW_IV]], [[N:%.*]]
-; CHECK-NEXT:    br i1 [[CMP]], label [[LOOP]], label [[EXIT:%.*]]
+; CHECK-NEXT:    br i1 false, label [[LOOP]], label [[EXIT:%.*]]
 ; CHECK:       exit:
 ; CHECK-NEXT:    ret void
 ;