[llvm] 9d9633f - [CVP] Simplify cmp of local phi node

[Nikita Popov via llvm-commits]

Author: Nikita Popov
Date: 2020-02-26T20:36:41+01:00
New Revision: 9d9633fb700d2c18b67b253a59145a543c6a37c4

URL: https://github.com/llvm/llvm-project/commit/9d9633fb700d2c18b67b253a59145a543c6a37c4
DIFF: https://github.com/llvm/llvm-project/commit/9d9633fb700d2c18b67b253a59145a543c6a37c4.diff

LOG: [CVP] Simplify cmp of local phi node

CVP currently does not simplify cmps with instructions in the same
block, because LVI getPredicateAt() currently does not provide
much useful information for that case (D69686 would change that,
but is stuck.) However, if the instruction is a Phi node, then
LVI can compute the result of the predicate by threading it into
the predecessor blocks, which allows it simplify some conditions
that nothing else can handle. Relevant code:
https://github.com/llvm/llvm-project/blob/6d6a4590c5d4c7fc7445d72fe685f966b0a8cafb/llvm/lib/Analysis/LazyValueInfo.cpp#L1904-L1927

Differential Revision: https://reviews.llvm.org/D72169

Added: 
    

Modified: 
    llvm/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp
    llvm/test/Transforms/CorrelatedValuePropagation/icmp.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp b/llvm/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp
index 3435bc7f5eaa..4d9883dd662e 100644
--- a/llvm/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp
+++ b/llvm/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp
@@ -310,9 +310,10 @@ static bool processCmp(CmpInst *Cmp, LazyValueInfo *LVI) {
   // the comparison is testing local values.  While LVI can sometimes reason
   // about such cases, it's not its primary purpose.  We do make sure to do
   // the block local query for uses from terminator instructions, but that's
-  // handled in the code for each terminator.
+  // handled in the code for each terminator. As an exception, we allow phi
+  // nodes, for which LVI can thread the condition into predecessors.
   auto *I = dyn_cast<Instruction>(Op0);
-  if (I && I->getParent() == Cmp->getParent())
+  if (I && I->getParent() == Cmp->getParent() && !isa<PHINode>(I))
     return false;
 
   LazyValueInfo::Tristate Result =

diff  --git a/llvm/test/Transforms/CorrelatedValuePropagation/icmp.ll b/llvm/test/Transforms/CorrelatedValuePropagation/icmp.ll
index b09d2fd607a3..fa2e6ff9af66 100644
--- a/llvm/test/Transforms/CorrelatedValuePropagation/icmp.ll
+++ b/llvm/test/Transforms/CorrelatedValuePropagation/icmp.ll
@@ -627,11 +627,9 @@ define void @test_cmp_phi(i8 %a) {
 ; CHECK:       loop:
 ; CHECK-NEXT:    [[P:%.*]] = phi i8 [ [[A]], [[ENTRY:%.*]] ], [ [[B:%.*]], [[LOOP]] ]
 ; CHECK-NEXT:    [[C1:%.*]] = icmp ne i8 [[P]], 0
-; CHECK-NEXT:    [[C2:%.*]] = icmp ne i8 [[P]], 2
-; CHECK-NEXT:    [[C3:%.*]] = and i1 [[C1]], [[C2]]
 ; CHECK-NEXT:    [[C4:%.*]] = call i1 @get_bool()
 ; CHECK-NEXT:    [[B]] = zext i1 [[C4]] to i8
-; CHECK-NEXT:    br i1 [[C3]], label [[LOOP]], label [[EXIT]]
+; CHECK-NEXT:    br i1 [[C1]], label [[LOOP]], label [[EXIT]]
 ; CHECK:       exit:
 ; CHECK-NEXT:    ret void
 ;