[GuardWidening] Ignore guards with trivial conditions

Guard widening should not spend efforts on dealing with guards with trivial true/false conditions.
Such guards can easily be eliminated by any further cleanup pass like instcombine. However we
should not unconditionally delete them because it may be profitable to widen other conditions
into such guards.

Differential Revision: https://reviews.llvm.org/D50247
Reviewed By: fedor.sergeev

llvm-svn: 340381

2 files changed, 32 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Scalar/GuardWidening.cpp b/llvm/lib/Transforms/Scalar/GuardWidening.cpp
index 9467e5ed651..68be46ec07b 100644
--- a/llvm/lib/Transforms/Scalar/GuardWidening.cpp
+++ b/llvm/lib/Transforms/Scalar/GuardWidening.cpp
@@ -347,6 +347,12 @@ bool GuardWideningImpl::eliminateGuardViaWidening(
     Instruction *GuardInst, const df_iterator<DomTreeNode *> &DFSI,
     const DenseMap<BasicBlock *, SmallVector<Instruction *, 8>> &
         GuardsInBlock, bool InvertCondition) {
+  // Ignore trivial true or false conditions. These instructions will be
+  // trivially eliminated by any cleanup pass. Do not erase them because other
+  // guards can possibly be widened into them.
+  if (isa<ConstantInt>(getCondition(GuardInst)))
+    return false;
+
   Instruction *BestSoFar = nullptr;
   auto BestScoreSoFar = WS_IllegalOrNegative;
   auto *GuardInstLoop = LI.getLoopFor(GuardInst->getParent());
diff --git a/llvm/test/Transforms/GuardWidening/basic.ll b/llvm/test/Transforms/GuardWidening/basic.ll
index 5b746a5d7dd..ab18ed7362a 100644
--- a/llvm/test/Transforms/GuardWidening/basic.ll
+++ b/llvm/test/Transforms/GuardWidening/basic.ll
@@ -379,3 +379,29 @@ left:
 right:
   ret void
 }
+
+; Make sure we do not widen guard by trivial true conditions into something.
+define void @f_15(i1 %cond_0, i1 %cond_1) {
+; CHECK-LABEL: @f_15(
+entry:
+; CHECK:  call void (i1, ...) @llvm.experimental.guard(i1 %cond_0) [ "deopt"() ]
+; CHECK:  call void (i1, ...) @llvm.experimental.guard(i1 true) [ "deopt"() ]
+; CHECK:  ret void
+
+  call void(i1, ...) @llvm.experimental.guard(i1 %cond_0) [ "deopt"() ]
+  call void(i1, ...) @llvm.experimental.guard(i1 true) [ "deopt"() ]
+  ret void
+}
+
+; Make sure we do not widen guard by trivial false conditions into something.
+define void @f_16(i1 %cond_0, i1 %cond_1) {
+; CHECK-LABEL: @f_16(
+entry:
+; CHECK:  call void (i1, ...) @llvm.experimental.guard(i1 %cond_0) [ "deopt"() ]
+; CHECK:  call void (i1, ...) @llvm.experimental.guard(i1 false) [ "deopt"() ]
+; CHECK:  ret void
+
+  call void(i1, ...) @llvm.experimental.guard(i1 %cond_0) [ "deopt"() ]
+  call void(i1, ...) @llvm.experimental.guard(i1 false) [ "deopt"() ]
+  ret void
+}