[PATCH] D16240: [InstCombine] Simplify a known nonzero incoming value of PHI

[Jun Bum Lim via llvm-commits]

junbuml created this revision.
junbuml added a reviewer: mcrosier.
junbuml added subscribers: gberry, mssimpso, bmakam, haicheng, llvm-commits.

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

http://reviews.llvm.org/D16240

Files:
  lib/Transforms/InstCombine/InstCombinePHI.cpp
  test/Transforms/InstCombine/phi.ll

Index: test/Transforms/InstCombine/phi.ll
===================================================================
--- test/Transforms/InstCombine/phi.ll
+++ test/Transforms/InstCombine/phi.ll
@@ -760,3 +760,22 @@
 ; CHECK-NEXT: ret i1 %[[RES]]
 }
 
+; CHECK-LABEL: @phi_knownnonzero
+; CHECK-NOT: select
+define i1 @phi_knownnonzero(i32 %n, i32 %s, i32* nocapture readonly %P) {
+entry:
+  %tobool = icmp slt  i32 %n, %s
+  br i1 %tobool, label %if.end, label %if.then
+
+if.then:                                          ; preds = %entry
+  %0 = load i32, i32* %P
+  %cmp = icmp eq i32 %n, %0
+  %1 = select i1 %cmp, i32 1, i32 2
+  br label %if.end
+
+if.end:                                           ; preds = %entry, %if.then
+  %a.0 = phi i32 [ %1,  %if.then ], [ %n, %entry ]
+  %cmp1 = icmp eq i32 %a.0, 0
+  ret i1  %cmp1
+}
+
Index: lib/Transforms/InstCombine/InstCombinePHI.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombinePHI.cpp
+++ lib/Transforms/InstCombine/InstCombinePHI.cpp
@@ -16,6 +16,8 @@
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/Analysis/InstructionSimplify.h"
 #include "llvm/Transforms/Utils/Local.h"
+#include "llvm/Analysis/ValueTracking.h"
+#include "llvm/IR/PatternMatch.h"
 using namespace llvm;
 
 #define DEBUG_TYPE "instcombine"
@@ -919,6 +921,27 @@
         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
+    ICmpInst *CmpInst = dyn_cast<ICmpInst>(PHIUser);
+    // FIXME: To be simple, handle only interger type for now.
+    if (CmpInst && isa<IntegerType>(PN.getType()) && CmpInst->isEquality() &&
+        (match(CmpInst->getOperand(1), PatternMatch::m_Zero()) ||
+         match(CmpInst->getOperand(0), PatternMatch::m_Zero()))) {
+      for (unsigned i = 0, e = PN.getNumIncomingValues(); i != e; ++i) {
+        Value *VA = PN.getIncomingValue(i);
+        if (!isa<ConstantInt>(VA) && VA->hasOneUse() &&
+            isKnownNonZero(VA, DL, 0, AC, &PN, DT))
+          PN.setIncomingValue(i, ConstantInt::get(PN.getType(), 1));
+      }
+    }
   }
 
   // We sometimes end up with phi cycles that non-obviously end up being the


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