[llvm] c7f7f60 - [ValueTracking] Handle bitcasts between vec-int-ptr in `isKnownNonZero`

[Noah Goldstein via llvm-commits]

Author: Noah Goldstein
Date: 2023-04-30T10:06:45-05:00
New Revision: c7f7f601f2c86203f8659e0a765549853f3c2a85

URL: https://github.com/llvm/llvm-project/commit/c7f7f601f2c86203f8659e0a765549853f3c2a85
DIFF: https://github.com/llvm/llvm-project/commit/c7f7f601f2c86203f8659e0a765549853f3c2a85.diff

LOG: [ValueTracking] Handle bitcasts between vec-int-ptr in `isKnownNonZero`

We where missing these cases so something like:
`(bitcast to i32 (or v216 x, <2, 1>))`

would not be found to be non-zero.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D149409

Added: 
    

Modified: 
    llvm/lib/Analysis/ValueTracking.cpp
    llvm/test/Analysis/ValueTracking/known-non-zero.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 1be618c202f7..9fb044b74580 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -2741,10 +2741,39 @@ bool isKnownNonZero(const Value *V, const APInt &DemandedElts, unsigned Depth,
     if (I->getType()->isPointerTy())
       return isGEPKnownNonNull(cast<GEPOperator>(I), Depth, Q);
     break;
-  case Instruction::BitCast:
-    if (I->getType()->isPointerTy())
+  case Instruction::BitCast: {
+    // We need to be a bit careful here. We can only peek through the bitcast
+    // if the scalar size of elements in the operand are smaller than and a
+    // multiple of the size they are casting too. Take three cases:
+    //
+    // 1) Unsafe:
+    //        bitcast <2 x i16> %NonZero to <4 x i8>
+    //
+    //    %NonZero can have 2 non-zero i16 elements, but isKnownNonZero on a
+    //    <4 x i8> requires that all 4 i8 elements be non-zero which isn't
+    //    guranteed (imagine just sign bit set in the 2 i16 elements).
+    //
+    // 2) Unsafe:
+    //        bitcast <4 x i3> %NonZero to <3 x i4>
+    //
+    //    Even though the scalar size of the src (`i3`) is smaller than the
+    //    scalar size of the dst `i4`, because `i3` is not a multiple of `i4`
+    //    its possible for the `3 x i4` elements to be zero because there are
+    //    some elements in the destination that don't contain any full src
+    //    element.
+    //
+    // 3) Safe:
+    //        bitcast <4 x i8> %NonZero to <2 x i16>
+    //
+    //    This is always safe as non-zero in the 4 i8 elements implies
+    //    non-zero in the combination of any two adjacent ones. Since i8 is a
+    //    multiple of i16, each i16 is guranteed to have 2 full i8 elements.
+    //    This all implies the 2 i16 elements are non-zero.
+    Type *FromTy = I->getOperand(0)->getType();
+    if ((FromTy->isIntOrIntVectorTy() || FromTy->isPtrOrPtrVectorTy()) &&
+        (BitWidth % getBitWidth(FromTy->getScalarType(), Q.DL)) == 0)
       return isKnownNonZero(I->getOperand(0), Depth, Q);
-    break;
+  } break;
   case Instruction::IntToPtr:
     // Note that we have to take special care to avoid looking through
     // truncating casts, e.g., int2ptr/ptr2int with appropriate sizes, as well

diff  --git a/llvm/test/Analysis/ValueTracking/known-non-zero.ll b/llvm/test/Analysis/ValueTracking/known-non-zero.ll
index 9fad0f82a625..2ae4ba08d947 100644
--- a/llvm/test/Analysis/ValueTracking/known-non-zero.ll
+++ b/llvm/test/Analysis/ValueTracking/known-non-zero.ll
@@ -596,11 +596,7 @@ define i1 @fshl_non_zero_fail(i8 %x, i8 %y, i8 %z, i8 %w) {
 
 define i1 @bitcast_nonzero(<2 x i8> %xx, i16 %ind) {
 ; CHECK-LABEL: @bitcast_nonzero(
-; CHECK-NEXT:    [[XA:%.*]] = add nuw nsw <2 x i8> [[XX:%.*]], <i8 1, i8 1>
-; CHECK-NEXT:    [[X:%.*]] = bitcast <2 x i8> [[XA]] to i16
-; CHECK-NEXT:    [[Z:%.*]] = or i16 [[X]], [[IND:%.*]]
-; CHECK-NEXT:    [[R:%.*]] = icmp eq i16 [[Z]], 0
-; CHECK-NEXT:    ret i1 [[R]]
+; CHECK-NEXT:    ret i1 false
 ;
   %xa = add nuw nsw <2 x i8> %xx, <i8 1, i8 1>
   %x = bitcast <2 x i8> %xa to i16
@@ -641,11 +637,7 @@ define <2 x i1> @bitcast_fail_nonzero_int_to_vec(i16 %xx, <2 x i8> %ind) {
 
 define <2 x i1> @bitcast_veci8_to_veci16(<4 x i8> %xx, <2 x i16> %ind) {
 ; CHECK-LABEL: @bitcast_veci8_to_veci16(
-; CHECK-NEXT:    [[XA:%.*]] = add nuw nsw <4 x i8> [[XX:%.*]], <i8 1, i8 1, i8 1, i8 1>
-; CHECK-NEXT:    [[X:%.*]] = bitcast <4 x i8> [[XA]] to <2 x i16>
-; CHECK-NEXT:    [[Z:%.*]] = or <2 x i16> [[X]], [[IND:%.*]]
-; CHECK-NEXT:    [[R:%.*]] = icmp eq <2 x i16> [[Z]], zeroinitializer
-; CHECK-NEXT:    ret <2 x i1> [[R]]
+; CHECK-NEXT:    ret <2 x i1> zeroinitializer
 ;
   %xa = add nuw nsw <4 x i8> %xx, <i8 1, i8 1, i8 1, i8 1>
   %x = bitcast <4 x i8> %xa to <2 x i16>