[ValueTracking] Improve isKnownNonZero for PHI of non-zero constants

[junbuml]

Summary

It is clear that a PHI is a non-zero if all incoming values are non-zero constants.

[sanjoy]

Minor comments inline.

lib/Analysis/ValueTracking.cpp

2065

A cleaner way of writing this would be:

// llvm::all_of from STLExtras.h
bool AllNonZeroConstants = all_of(PN->operands(), [](Value *V) {
  return isa<ConstantInt>(V) && !cast<ConstantInt>(V)->isZeroValue();
});
if (AllNonZeroConstants)
  return true;

test/Transforms/InstCombine/phi.ll

768

Minor: you can simplify the branch to br i1 %c, label %if.then, label %if.else.

774

Minor: I'd add a call to an unknown function in one of the if branches. Without it simplifycfg changes the PHI to a select (and elides the control flow), after which LLVM is able to optimize this function in -O3, but with a call void @foo() in one of the branches, the phi->select conversion does not happen, and this test case demonstrates a truly missed optimization in LLVM today.

[junbuml]

Thanks Sanjoy for the review.

[junbuml]

Fixed Sanjoy's comment again.

[junbuml]

Landed in r259370. Thanks Sanjoy for the review.

lib/Analysis/ValueTracking.cpp

if (PN->getNumIncomingValues() == 2) {
           ConstantInt *X;
           if ((match(Induction, m_NSWAdd(m_Specific(PN), m_ConstantInt(X))) ||
                match(Induction, m_NUWAdd(m_Specific(PN), m_ConstantInt(X)))) &&
               !X->isNegative())
             return true;
         }
       }
     }
+    // Check if all incoming values are non-zero constant.
+    bool AllNonZeroConstants = all_of(PN->operands(), [](Value *V) {
+      return isa<ConstantInt>(V) && !cast<ConstantInt>(V)->isZeroValue();
+    });
+    if (AllNonZeroConstants)
+      return true;
   }
 
   if (!BitWidth) return false;
   APInt KnownZero(BitWidth, 0);
   APInt KnownOne(BitWidth, 0);
   computeKnownBits(V, KnownZero, KnownOne, Depth, Q);
   return KnownOne != 0;
 }

test/Transforms/InstCombine/phi.ll

epilog:
   %tobool = icmp ne i8 %conditionMet, 0
   ret i1 %tobool
 
 ; CHECK-LABEL: @PR24766_two_constants_two_var(
 ; CHECK: %[[RES:.*]] = phi i1 [ false, %entry ], [ %cmp2, %sw2 ], [ %cmp1, %sw1 ], [ true, %sw3 ]
 ; CHECK-NEXT: ret i1 %[[RES]]
 }
 
+; CHECK-LABEL: phi_allnonzeroconstant
+; CHECK-NOT: phi i32
+; CHECK: ret i1 false
+define i1 @phi_allnonzeroconstant(i1 %c, i32 %a, i32 %b) {
+entry:
+  br i1 %c, label %if.then, label %if.else
+
+if.then:                                          ; preds = %entry
+  br label %if.end
+
+if.else:                                          ; preds = %entry
+  call void @dummy()
+  br label %if.end
+
+if.end:                                           ; preds = %if.else, %if.then
+  %x.0 = phi i32 [ 1, %if.then ], [ 2, %if.else ]
+  %or = or i32 %x.0, %a
+  %cmp1 = icmp eq i32 %or, 0
+  ret i1 %cmp1
+}
+
+declare void @dummy()
+