[InstCombine] Simplify a known nonzero incoming value of PHI
Summary:
When a PHI is used only to be compared with zero, it is possible to replace an
incoming value with any non-zero constant if the incoming value can be proved as
a known nonzero value. For example, in below code, we can replace the incoming value %v with
any non-zero constant based on the fact that the PHI is only used to be compared with zero
and %v is a known non-zero value:
%v = select %cond, 1, 2
%p = phi [%v, BB] ...
%c = icmp eq, %p, 0
Reviewers: mcrosier, jmolloy, sanjoy
Subscribers: hfinkel, mcrosier, majnemer, llvm-commits, haicheng, bmakam, mssimpso, gberry
Differential Revision: http://reviews.llvm.org/D16240
llvm-svn: 260530
diff --git a/llvm/lib/Transforms/InstCombine/InstCombinePHI.cpp b/llvm/lib/Transforms/InstCombine/InstCombinePHI.cpp
index f8c6fa5..138a95c 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombinePHI.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombinePHI.cpp
@@ -15,8 +15,11 @@
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/Analysis/InstructionSimplify.h"
+#include "llvm/Analysis/ValueTracking.h"
+#include "llvm/IR/PatternMatch.h"
#include "llvm/Transforms/Utils/Local.h"
using namespace llvm;
+using namespace llvm::PatternMatch;
#define DEBUG_TYPE "instcombine"
@@ -641,6 +644,16 @@
return true;
}
+/// Return an existing non-zero constant if this phi node has, otherwise ruturn
+/// constant 1.
+static ConstantInt *GetAnyNonZeroConstInt(PHINode &PN) {
+ assert(isa<IntegerType>(PN.getType()) && "Expect only intger type phi");
+ for (Value *V : PN.operands())
+ if (auto *ConstVA = dyn_cast<ConstantInt>(V))
+ if (!ConstVA->isZeroValue())
+ return ConstVA;
+ return ConstantInt::get(cast<IntegerType>(PN.getType()), 1);
+}
namespace {
struct PHIUsageRecord {
@@ -919,6 +932,29 @@
PHIUser->user_back() == &PN) {
return replaceInstUsesWith(PN, UndefValue::get(PN.getType()));
}
+ // When a PHI is used only to be compared with zero, it is safe to replace
+ // an incoming value proved as known nonzero with any non-zero constant.
+ // For example, in below code, the incoming value %v can be replaced with
+ // any non-zero constant based on the fact that the PHI is only used to be
+ // compared with zero and %v is a known non-zero value:
+ // %v = select %cond, 1, 2
+ // %p = phi [%v, BB] ...
+ // icmp eq, %p, 0
+ auto *CmpInst = dyn_cast<ICmpInst>(PHIUser);
+ // FIXME: To be simple, handle only integer type for now.
+ if (CmpInst && isa<IntegerType>(PN.getType()) && CmpInst->isEquality() &&
+ match(CmpInst->getOperand(1), m_Zero())) {
+ ConstantInt *NonZeroConst = nullptr;
+ for (unsigned i = 0, e = PN.getNumIncomingValues(); i != e; ++i) {
+ Instruction *CtxI = PN.getIncomingBlock(i)->getTerminator();
+ Value *VA = PN.getIncomingValue(i);
+ if (isKnownNonZero(VA, DL, 0, AC, CtxI, DT)) {
+ if (!NonZeroConst)
+ NonZeroConst = GetAnyNonZeroConstInt(PN);
+ PN.setIncomingValue(i, NonZeroConst);
+ }
+ }
+ }
}
// We sometimes end up with phi cycles that non-obviously end up being the