[ValueTracking] Teach computeKnownBits for PHI nodes to compute sign bit for a recurrence with a NSW addition.

If a operation for a recurrence is an addition with no signed wrap and both input sign bits are 0, then the result sign bit must also be 0. Similar for the negative case.

I found this deficiency while playing around with a loop in the x86 backend that contained a signed division that could be optimized into an unsigned division if we could prove both inputs were positive. One of them being the loop induction variable. With this patch we can perform the conversion for this case. One of the test cases here is a contrived variation of the loop I was looking at.

Differential revision: http://reviews.llvm.org/D21493

llvm-svn: 274098

3 files changed, 128 insertions, 1 deletions

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 16dd3078160..78ec2201fd5 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -1240,6 +1240,18 @@ static void computeKnownBitsFromOperator(Operator *I, APInt &KnownZero,
           KnownZero = APInt::getLowBitsSet(BitWidth,
                                            std::min(KnownZero2.countTrailingOnes(),
                                                     KnownZero3.countTrailingOnes()));
+
+          // If the operation is an addition that can't have signed overflow,
+          // then the sign bit is known to be zero if both input sign bits
+          // are zero. Similar for two negative inputs.
+          if (Opcode == Instruction::Add &&
+              cast<OverflowingBinaryOperator>(LU)->hasNoSignedWrap()) {
+            if (KnownZero2.isNegative() && KnownZero3.isNegative())
+              KnownZero.setBit(BitWidth-1);
+            if (KnownOne2.isNegative() && KnownOne3.isNegative())
+              KnownOne.setBit(BitWidth-1);
+          }
+
           break;
         }
       }
diff --git a/llvm/test/Transforms/BBVectorize/loop1.ll b/llvm/test/Transforms/BBVectorize/loop1.ll
index 70a5def4222..445dec1516d 100644
--- a/llvm/test/Transforms/BBVectorize/loop1.ll
+++ b/llvm/test/Transforms/BBVectorize/loop1.ll
@@ -83,7 +83,7 @@ for.body:                                         ; preds = %for.body, %entry
 ; CHECK-UNRL: %add12 = fadd <2 x double> %add7, %mul11
 ; CHECK-UNRL: %4 = bitcast double* %arrayidx14 to <2 x double>*
 ; CHECK-UNRL: store <2 x double> %add12, <2 x double>* %4, align 8
-; CHECK-UNRL: %indvars.iv.next.1 = add nsw i64 %indvars.iv, 2
+; CHECK-UNRL: %indvars.iv.next.1 = add nuw nsw i64 %indvars.iv, 2
 ; CHECK-UNRL: %lftr.wideiv.1 = trunc i64 %indvars.iv.next.1 to i32
 ; CHECK-UNRL: %exitcond.1 = icmp eq i32 %lftr.wideiv.1, 10
 ; CHECK-UNRL: br i1 %exitcond.1, label %for.end, label %for.body
diff --git a/llvm/test/Transforms/InstCombine/phi.ll b/llvm/test/Transforms/InstCombine/phi.ll
index c417737fdf2..9b547361f45 100644
--- a/llvm/test/Transforms/InstCombine/phi.ll
+++ b/llvm/test/Transforms/InstCombine/phi.ll
@@ -879,3 +879,118 @@ if.end:                                           ; preds = %entry, %if.then
   %cmp1 = icmp ne i32 %a.0, 0
   ret i1  %cmp1
 }
+
+
+; This test makes sure we can determine that the inputs to the sdiv in the loop
+; are non-negative and can become a udiv. This requires that we recognize that
+; the loop induction can never have its sign bit set.
+;
+; CHECK-LABEL: @phi_nsw_induction_sdiv_udiv
+; CHECK: udiv
+; CHECK: udiv
+define i32 @phi_nsw_induction_sdiv_udiv(i32 %NumElts, i32 %ScalarSize) {
+entry:
+  %div = udiv i32 128, %ScalarSize
+  br label %for.cond
+
+for.cond:                                         ; preds = %for.inc, %entry
+  %Sum.0 = phi i32 [ 0, %entry ], [ %add, %for.inc ]
+  %i.0 = phi i32 [ 0, %entry ], [ %inc, %for.inc ]
+  %cmp = icmp ne i32 %i.0, %NumElts
+  br i1 %cmp, label %for.body, label %for.end
+
+for.body:                                         ; preds = %for.cond
+  ; this should become a udiv
+  %div1 = sdiv i32 %i.0, %div
+  %add = add nsw i32 %Sum.0, %div1
+  br label %for.inc
+
+for.inc:                                          ; preds = %for.body
+  %inc = add nsw i32 %i.0, 1
+  br label %for.cond
+
+for.end:                                          ; preds = %for.cond
+  ret i32 %Sum.0
+}
+
+
+; CHECK-LABEL: test_positive_nsw_recurrence
+; CHECK-NOT: bar
+; CHECK: foo
+; CHECK-NOT: bar
+; CHECK: ret
+define void @test_positive_nsw_recurrence(i32 %N) {
+entry:
+  br label %for.cond
+
+for.cond:                                         ; preds = %for.inc, %entry
+  %i.0 = phi i32 [ 0, %entry ], [ %inc, %for.inc ]
+  %cmp = icmp ne i32 %i.0, %N
+  br i1 %cmp, label %for.body, label %for.end
+
+for.body:                                         ; preds = %for.cond
+  %and = and i32 %i.0, -2147483648
+  %tobool = icmp ne i32 %and, 0
+  br i1 %tobool, label %if.then, label %if.else
+
+if.then:                                          ; preds = %for.body
+  ; this call should be deleted as %i.0 can never be negative due to no signed wrap
+  call void @bar()
+  br label %if.end
+
+if.else:                                          ; preds = %for.body
+  call void @foo()
+  br label %if.end
+
+if.end:                                           ; preds = %if.else, %if.then
+  br label %for.inc
+
+for.inc:                                          ; preds = %if.end
+  %inc = add nsw i32 %i.0, 1
+  br label %for.cond
+
+for.end:                                          ; preds = %for.cond
+  ret void
+}
+
+; CHECK-LABEL: test_negative_nsw_recurrence
+; CHECK-NOT: foo
+; CHECK: bar
+; CHECK-NOT: foo
+; CHECK: ret
+define void @test_negative_nsw_recurrence(i32 %N) {
+entry:
+  br label %for.cond
+
+for.cond:                                         ; preds = %for.inc, %entry
+  %i.0 = phi i32 [ -1, %entry ], [ %inc, %for.inc ]
+  %cmp = icmp ne i32 %i.0, %N
+  br i1 %cmp, label %for.body, label %for.end
+
+for.body:                                         ; preds = %for.cond
+  %and = and i32 %i.0, -2147483648
+  %tobool = icmp ne i32 %and, 0
+  br i1 %tobool, label %if.then, label %if.else
+
+if.then:                                          ; preds = %for.body
+  call void @bar()
+  br label %if.end
+
+if.else:                                          ; preds = %for.body
+  ; this call should be deleted as %i.0 can never be positive due to no signed wrap
+  call void @foo()
+  br label %if.end
+
+if.end:                                           ; preds = %if.else, %if.then
+  br label %for.inc
+
+for.inc:                                          ; preds = %if.end
+  %inc = add nsw i32 %i.0, -1
+  br label %for.cond
+
+for.end:                                          ; preds = %for.cond
+  ret void
+}
+
+declare void @bar()
+declare void @foo()