[Reland][ValueTracking] Improve Bitcast handling to match SDAG (#145223)

Fixes #125228

---------

Co-authored-by: Simon Pilgrim <[email protected]>

Author: abhishek-kaushik22

llvm/lib/Analysis/ValueTracking.cpp

@@ -1351,6 +1351,8 @@ static void computeKnownBitsFromOperator(const Operator *I,
         isa<ScalableVectorType>(I->getType()))
       break;
 
+    unsigned NumElts = DemandedElts.getBitWidth();
+    bool IsLE = Q.DL.isLittleEndian();
     // Look through a cast from narrow vector elements to wider type.
     // Examples: v4i32 -> v2i64, v3i8 -> v24
     unsigned SubBitWidth = SrcVecTy->getScalarSizeInBits();
@@ -1369,7 +1371,6 @@ static void computeKnownBitsFromOperator(const Operator *I,
       //
       // The known bits of each sub-element are then inserted into place
       // (dependent on endian) to form the full result of known bits.
-      unsigned NumElts = DemandedElts.getBitWidth();
       unsigned SubScale = BitWidth / SubBitWidth;
       APInt SubDemandedElts = APInt::getZero(NumElts * SubScale);
       for (unsigned i = 0; i != NumElts; ++i) {
@@ -1381,10 +1382,32 @@ static void computeKnownBitsFromOperator(const Operator *I,
       for (unsigned i = 0; i != SubScale; ++i) {
         computeKnownBits(I->getOperand(0), SubDemandedElts.shl(i), KnownSrc, Q,
                          Depth + 1);
-        unsigned ShiftElt = Q.DL.isLittleEndian() ? i : SubScale - 1 - i;
+        unsigned ShiftElt = IsLE ? i : SubScale - 1 - i;
         Known.insertBits(KnownSrc, ShiftElt * SubBitWidth);
       }
     }
+    // Look through a cast from wider vector elements to narrow type.
+    // Examples: v2i64 -> v4i32
+    if (SubBitWidth % BitWidth == 0) {
+      unsigned SubScale = SubBitWidth / BitWidth;
+      KnownBits KnownSrc(SubBitWidth);
+      APInt SubDemandedElts =
+          APIntOps::ScaleBitMask(DemandedElts, NumElts / SubScale);
+      computeKnownBits(I->getOperand(0), SubDemandedElts, KnownSrc, Q,
+                       Depth + 1);
+
+      Known.Zero.setAllBits();
+      Known.One.setAllBits();
+      for (unsigned i = 0; i != NumElts; ++i) {
+        if (DemandedElts[i]) {
+          unsigned Shifts = IsLE ? i : NumElts - 1 - i;
+          unsigned Offset = (Shifts % SubScale) * BitWidth;
+          Known = Known.intersectWith(KnownSrc.extractBits(BitWidth, Offset));
+          if (Known.isUnknown())
+            break;
+        }
+      }
+    }
     break;
   }
   case Instruction::SExt: {

llvm/test/Transforms/InstCombine/X86/x86-vector-shifts.ll

@@ -3732,7 +3732,6 @@ define <4 x i64> @test_avx2_psrl_0() {
   ret <4 x i64> %16
 }
 
-; FIXME: Failure to peek through bitcasts to ensure psllq shift amount is within bounds.
 define <2 x i64> @PR125228(<2 x i64> %v, <2 x i64> %s) {
 ; CHECK-LABEL: @PR125228(
 ; CHECK-NEXT:    [[MASK:%.*]] = and <2 x i64> [[S:%.*]], splat (i64 63)
@@ -3741,7 +3740,8 @@ define <2 x i64> @PR125228(<2 x i64> %v, <2 x i64> %s) {
 ; CHECK-NEXT:    [[CAST:%.*]] = bitcast <2 x i64> [[MASK]] to <16 x i8>
 ; CHECK-NEXT:    [[PSRLDQ:%.*]] = shufflevector <16 x i8> [[CAST]], <16 x i8> poison, <16 x i32> <i32 8, i32 9, i32 10, i32 11, i32 12, i32 13, i32 14, i32 15, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison>
 ; CHECK-NEXT:    [[CAST3:%.*]] = bitcast <16 x i8> [[PSRLDQ]] to <2 x i64>
-; CHECK-NEXT:    [[SLL1:%.*]] = call <2 x i64> @llvm.x86.sse2.psll.q(<2 x i64> [[V]], <2 x i64> [[CAST3]])
+; CHECK-NEXT:    [[TMP2:%.*]] = shufflevector <2 x i64> [[CAST3]], <2 x i64> poison, <2 x i32> zeroinitializer
+; CHECK-NEXT:    [[SLL1:%.*]] = shl <2 x i64> [[V]], [[TMP2]]
 ; CHECK-NEXT:    [[SHUFP_UNCASTED:%.*]] = shufflevector <2 x i64> [[SLL0]], <2 x i64> [[SLL1]], <2 x i32> <i32 0, i32 3>
 ; CHECK-NEXT:    ret <2 x i64> [[SHUFP_UNCASTED]]
 ;

llvm/test/Transforms/InstCombine/bitcast-known-bits.ll

@@ -12,8 +12,7 @@ define <16 x i8> @knownbits_bitcast_masked_shift(<16 x i8> %arg1, <16 x i8> %arg
 ; CHECK-NEXT:    [[BITCAST4:%.*]] = bitcast <16 x i8> [[OR]] to <8 x i16>
 ; CHECK-NEXT:    [[SHL5:%.*]] = shl nuw <8 x i16> [[BITCAST4]], splat (i16 2)
 ; CHECK-NEXT:    [[BITCAST6:%.*]] = bitcast <8 x i16> [[SHL5]] to <16 x i8>
-; CHECK-NEXT:    [[AND7:%.*]] = and <16 x i8> [[BITCAST6]], splat (i8 -52)
-; CHECK-NEXT:    ret <16 x i8> [[AND7]]
+; CHECK-NEXT:    ret <16 x i8> [[BITCAST6]]
 ;
   %and = and <16 x i8> %arg1, splat (i8 3)
   %and3 = and <16 x i8> %arg2, splat (i8 48)
@@ -33,8 +32,7 @@ define <16 x i8> @knownbits_shuffle_masked_nibble_shift(<16 x i8> %arg)  {
 ; CHECK-NEXT:    [[BITCAST1:%.*]] = bitcast <16 x i8> [[SHUFFLEVECTOR]] to <8 x i16>
 ; CHECK-NEXT:    [[SHL:%.*]] = shl nuw <8 x i16> [[BITCAST1]], splat (i16 4)
 ; CHECK-NEXT:    [[BITCAST2:%.*]] = bitcast <8 x i16> [[SHL]] to <16 x i8>
-; CHECK-NEXT:    [[AND3:%.*]] = and <16 x i8> [[BITCAST2]], splat (i8 -16)
-; CHECK-NEXT:    ret <16 x i8> [[AND3]]
+; CHECK-NEXT:    ret <16 x i8> [[BITCAST2]]
 ;
   %and = and <16 x i8> %arg, splat (i8 15)
   %shufflevector = shufflevector <16 x i8> %and, <16 x i8> poison, <16 x i32> <i32 1, i32 0, i32 3, i32 2, i32 5, i32 4, i32 7, i32 6, i32 9, i32 8, i32 11, i32 10, i32 13, i32 12, i32 15, i32 14>
@@ -53,8 +51,7 @@ define <16 x i8> @knownbits_reverse_shuffle_masked_shift(<16 x i8> %arg)  {
 ; CHECK-NEXT:    [[BITCAST1:%.*]] = bitcast <16 x i8> [[SHUFFLEVECTOR]] to <8 x i16>
 ; CHECK-NEXT:    [[SHL:%.*]] = shl nuw <8 x i16> [[BITCAST1]], splat (i16 4)
 ; CHECK-NEXT:    [[BITCAST2:%.*]] = bitcast <8 x i16> [[SHL]] to <16 x i8>
-; CHECK-NEXT:    [[AND3:%.*]] = and <16 x i8> [[BITCAST2]], splat (i8 -16)
-; CHECK-NEXT:    ret <16 x i8> [[AND3]]
+; CHECK-NEXT:    ret <16 x i8> [[BITCAST2]]
 ;
   %and = and <16 x i8> %arg, splat (i8 15)
   %shufflevector = shufflevector <16 x i8> %and, <16 x i8> poison, <16 x i32> <i32 3, i32 2, i32 1, i32 0, i32 7, i32 6, i32 5, i32 4, i32 11, i32 10, i32 9, i32 8, i32 15, i32 14, i32 13, i32 12>
@@ -70,8 +67,7 @@ define <16 x i8> @knownbits_extract_bit(<8 x i16> %arg)  {
 ; CHECK-SAME: <8 x i16> [[ARG:%.*]]) {
 ; CHECK-NEXT:    [[LSHR:%.*]] = lshr <8 x i16> [[ARG]], splat (i16 15)
 ; CHECK-NEXT:    [[BITCAST1:%.*]] = bitcast <8 x i16> [[LSHR]] to <16 x i8>
-; CHECK-NEXT:    [[AND:%.*]] = and <16 x i8> [[BITCAST1]], splat (i8 1)
-; CHECK-NEXT:    ret <16 x i8> [[AND]]
+; CHECK-NEXT:    ret <16 x i8> [[BITCAST1]]
 ;
   %lshr = lshr <8 x i16> %arg, splat (i16 15)
   %bitcast1 = bitcast <8 x i16> %lshr to <16 x i8>
@@ -88,7 +84,8 @@ define { i32, i1 } @knownbits_popcount_add_with_overflow(<2 x i64> %arg1, <2 x i
 ; CHECK-NEXT:    [[CALL9:%.*]] = tail call range(i64 0, 65) <2 x i64> @llvm.ctpop.v2i64(<2 x i64> [[ARG2]])
 ; CHECK-NEXT:    [[BITCAST10:%.*]] = bitcast <2 x i64> [[CALL9]] to <4 x i32>
 ; CHECK-NEXT:    [[EXTRACTELEMENT11:%.*]] = extractelement <4 x i32> [[BITCAST10]], i64 0
-; CHECK-NEXT:    [[TMP1:%.*]] = tail call { i32, i1 } @llvm.uadd.with.overflow.i32(i32 [[EXTRACTELEMENT]], i32 [[EXTRACTELEMENT11]])
+; CHECK-NEXT:    [[CALL12:%.*]] = add nuw nsw i32 [[EXTRACTELEMENT]], [[EXTRACTELEMENT11]]
+; CHECK-NEXT:    [[TMP1:%.*]] = insertvalue { i32, i1 } { i32 poison, i1 false }, i32 [[CALL12]], 0
 ; CHECK-NEXT:    ret { i32, i1 } [[TMP1]]
 ;
   %call = tail call <2 x i64> @llvm.ctpop.v2i64(<2 x i64> %arg1)
@@ -110,11 +107,7 @@ define <16 x i8> @knownbits_shuffle_add_shift_v32i8(<16 x i8> %arg1, <8 x i16> %
 ; CHECK-NEXT:    [[BITCAST11:%.*]] = bitcast <8 x i16> [[SHL10]] to <16 x i8>
 ; CHECK-NEXT:    [[ADD12:%.*]] = add <16 x i8> [[BITCAST11]], [[BITCAST7]]
 ; CHECK-NEXT:    [[ADD14:%.*]] = add <16 x i8> [[ADD12]], [[ARG1]]
-; CHECK-NEXT:    [[BITCAST14:%.*]] = bitcast <16 x i8> [[ADD12]] to <8 x i16>
-; CHECK-NEXT:    [[SHL15:%.*]] = shl <8 x i16> [[BITCAST14]], splat (i16 8)
-; CHECK-NEXT:    [[BITCAST16:%.*]] = bitcast <8 x i16> [[SHL15]] to <16 x i8>
-; CHECK-NEXT:    [[ADD13:%.*]] = add <16 x i8> [[ADD14]], [[BITCAST16]]
-; CHECK-NEXT:    ret <16 x i8> [[ADD13]]
+; CHECK-NEXT:    ret <16 x i8> [[ADD14]]
 ;
   %shl6 = shl <8 x i16> %arg2, splat (i16 8)
   %bitcast7 = bitcast <8 x i16> %shl6 to <16 x i8>

llvm/test/Transforms/InstSimplify/shift-knownbits.ll

@@ -499,3 +499,29 @@ define <1 x i64> @bitcast_noshift_vector_wrong_type(<2 x float> %v1, <1 x i64> %
   %r = shl <1 x i64> %v2, %b
   ret <1 x i64> %r
 }
+
+; Test that verifies correct handling of known bits when bitcasting from a smaller vector
+; to a larger one (e.g., <2 x i32> to <8 x i8>). Previously, only the subscale portion
+; (e.g., 4 elements) was checked instead of the full demanded vector width (8 elements),
+; leading to incorrect known bits and removal of the `ashr` instruction.
+
+define <8 x i8> @bitcast_knownbits_subscale_miscompile(i32 %x) {
+; CHECK-LABEL: @bitcast_knownbits_subscale_miscompile(
+; CHECK-NEXT:    [[MASKED:%.*]] = and i32 [[X:%.*]], -256
+; CHECK-NEXT:    [[SETBITS:%.*]] = or i32 [[MASKED]], -16777216
+; CHECK-NEXT:    [[INSERT:%.*]] = insertelement <2 x i32> poison, i32 [[SETBITS]], i32 0
+; CHECK-NEXT:    [[SPLAT:%.*]] = shufflevector <2 x i32> [[INSERT]], <2 x i32> poison, <2 x i32> zeroinitializer
+; CHECK-NEXT:    [[VEC:%.*]] = bitcast <2 x i32> [[SPLAT]] to <8 x i8>
+; CHECK-NEXT:    [[SHUF:%.*]] = shufflevector <8 x i8> [[VEC]], <8 x i8> zeroinitializer, <8 x i32> <i32 7, i32 7, i32 7, i32 7, i32 0, i32 0, i32 0, i32 0>
+; CHECK-NEXT:    [[SHR:%.*]] = ashr <8 x i8> [[SHUF]], splat (i8 1)
+; CHECK-NEXT:    ret <8 x i8> [[SHR]]
+;
+  %masked = and i32 %x, u0xFFFFFF00
+  %setbits = or i32 %masked, u0xFF000000
+  %insert = insertelement <2 x i32> poison, i32 %setbits, i32 0
+  %splat = shufflevector <2 x i32> %insert, <2 x i32> poison, <2 x i32> splat (i32 0)
+  %vec = bitcast <2 x i32> %splat to <8 x i8>
+  %shuf = shufflevector <8 x i8> %vec, <8 x i8> zeroinitializer, <8 x i32> <i32 7, i32 7, i32 7, i32 7, i32 0, i32 0, i32 0, i32 0>
+  %shr = ashr <8 x i8> %shuf, splat (i8 1)
+  ret <8 x i8> %shr
+}