[InstCombine] try to narrow shifted bswap-of-zext (2nd try)

The first attempt at this missed a validity check.
This version includes a test of the narrow source
type for modulo-16-bits.

Original commit message:

This is the IR counterpart to 370ebc9
which provided a bswap narrowing fix for issue #53867.

Here we can be more general (although I'm not sure yet
what would happen for illegal types in codegen - too
rare to worry about?):
https://alive2.llvm.org/ce/z/3-CPfo

This will be more effective if we have moved the shift
after the bswap as proposed in D122010, but it is
independent of that patch.

Differential Revision: https://reviews.llvm.org/D122166

llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp

@@ -1173,6 +1173,22 @@ Instruction *InstCombinerImpl::visitLShr(BinaryOperator &I) {
         MulC->logBase2() == ShAmtC)
       return BinaryOperator::CreateAnd(X, ConstantInt::get(Ty, *MulC - 2));
 
+    // Try to narrow a bswap:
+    // (bswap (zext X)) >> C --> zext (bswap X >> C')
+    // In the case where the shift amount equals the bitwidth difference, the
+    // shift is eliminated.
+    if (match(Op0, m_OneUse(m_Intrinsic<Intrinsic::bswap>(
+                       m_OneUse(m_ZExt(m_Value(X))))))) {
+      // TODO: If the shift amount is less than the zext, we could shift left.
+      unsigned SrcWidth = X->getType()->getScalarSizeInBits();
+      unsigned WidthDiff = BitWidth - SrcWidth;
+      if (SrcWidth % 16 == 0 && ShAmtC >= WidthDiff) {
+        Value *NarrowSwap = Builder.CreateUnaryIntrinsic(Intrinsic::bswap, X);
+        Value *NewShift = Builder.CreateLShr(NarrowSwap, ShAmtC - WidthDiff);
+        return new ZExtInst(NewShift, Ty);
+      }
+    }
+
     // If the shifted-out value is known-zero, then this is an exact shift.
     if (!I.isExact() &&
         MaskedValueIsZero(Op0, APInt::getLowBitsSet(BitWidth, ShAmtC), 0, &I)) {

llvm/test/Transforms/InstCombine/lshr.ll

@@ -831,9 +831,8 @@ define i1 @icmp_sge(i32 %x, i32 %y) {
 
 define i32 @narrow_bswap(i16 %x) {
 ; CHECK-LABEL: @narrow_bswap(
-; CHECK-NEXT:    [[Z:%.*]] = zext i16 [[X:%.*]] to i32
-; CHECK-NEXT:    [[B:%.*]] = call i32 @llvm.bswap.i32(i32 [[Z]])
-; CHECK-NEXT:    [[S:%.*]] = lshr exact i32 [[B]], 16
+; CHECK-NEXT:    [[TMP1:%.*]] = call i16 @llvm.bswap.i16(i16 [[X:%.*]])
+; CHECK-NEXT:    [[S:%.*]] = zext i16 [[TMP1]] to i32
 ; CHECK-NEXT:    ret i32 [[S]]
 ;
   %z = zext i16 %x to i32
@@ -844,9 +843,8 @@ define i32 @narrow_bswap(i16 %x) {
 
 define i128 @narrow_bswap_extra_wide(i16 %x) {
 ; CHECK-LABEL: @narrow_bswap_extra_wide(
-; CHECK-NEXT:    [[Z:%.*]] = zext i16 [[X:%.*]] to i128
-; CHECK-NEXT:    [[B:%.*]] = call i128 @llvm.bswap.i128(i128 [[Z]])
-; CHECK-NEXT:    [[S:%.*]] = lshr exact i128 [[B]], 112
+; CHECK-NEXT:    [[TMP1:%.*]] = call i16 @llvm.bswap.i16(i16 [[X:%.*]])
+; CHECK-NEXT:    [[S:%.*]] = zext i16 [[TMP1]] to i128
 ; CHECK-NEXT:    ret i128 [[S]]
 ;
   %z = zext i16 %x to i128
@@ -855,6 +853,8 @@ define i128 @narrow_bswap_extra_wide(i16 %x) {
   ret i128 %s
 }
 
+; TODO: The bswap can be narrowed followed by shl.
+
 define i32 @narrow_bswap_undershift(i16 %x) {
 ; CHECK-LABEL: @narrow_bswap_undershift(
 ; CHECK-NEXT:    [[Z:%.*]] = zext i16 [[X:%.*]] to i32
@@ -870,9 +870,8 @@ define i32 @narrow_bswap_undershift(i16 %x) {
 
 define <2 x i64> @narrow_bswap_splat(<2 x i16> %x) {
 ; CHECK-LABEL: @narrow_bswap_splat(
-; CHECK-NEXT:    [[Z:%.*]] = zext <2 x i16> [[X:%.*]] to <2 x i64>
-; CHECK-NEXT:    [[B:%.*]] = call <2 x i64> @llvm.bswap.v2i64(<2 x i64> [[Z]])
-; CHECK-NEXT:    [[S:%.*]] = lshr exact <2 x i64> [[B]], <i64 48, i64 48>
+; CHECK-NEXT:    [[TMP1:%.*]] = call <2 x i16> @llvm.bswap.v2i16(<2 x i16> [[X:%.*]])
+; CHECK-NEXT:    [[S:%.*]] = zext <2 x i16> [[TMP1]] to <2 x i64>
 ; CHECK-NEXT:    ret <2 x i64> [[S]]
 ;
   %z = zext <2 x i16> %x to <2 x i64>
@@ -881,6 +880,8 @@ define <2 x i64> @narrow_bswap_splat(<2 x i16> %x) {
   ret <2 x i64> %s
 }
 
+; TODO: poison/undef in the shift amount is ok to propagate.
+
 define <2 x i64> @narrow_bswap_splat_poison_elt(<2 x i16> %x) {
 ; CHECK-LABEL: @narrow_bswap_splat_poison_elt(
 ; CHECK-NEXT:    [[Z:%.*]] = zext <2 x i16> [[X:%.*]] to <2 x i64>
@@ -896,9 +897,9 @@ define <2 x i64> @narrow_bswap_splat_poison_elt(<2 x i16> %x) {
 
 define <2 x i64> @narrow_bswap_overshift(<2 x i32> %x) {
 ; CHECK-LABEL: @narrow_bswap_overshift(
-; CHECK-NEXT:    [[Z:%.*]] = zext <2 x i32> [[X:%.*]] to <2 x i64>
-; CHECK-NEXT:    [[B:%.*]] = call <2 x i64> @llvm.bswap.v2i64(<2 x i64> [[Z]])
-; CHECK-NEXT:    [[S:%.*]] = lshr <2 x i64> [[B]], <i64 48, i64 48>
+; CHECK-NEXT:    [[TMP1:%.*]] = call <2 x i32> @llvm.bswap.v2i32(<2 x i32> [[X:%.*]])
+; CHECK-NEXT:    [[TMP2:%.*]] = lshr <2 x i32> [[TMP1]], <i32 16, i32 16>
+; CHECK-NEXT:    [[S:%.*]] = zext <2 x i32> [[TMP2]] to <2 x i64>
 ; CHECK-NEXT:    ret <2 x i64> [[S]]
 ;
   %z = zext <2 x i32> %x to <2 x i64>
@@ -909,9 +910,9 @@ define <2 x i64> @narrow_bswap_overshift(<2 x i32> %x) {
 
 define i128 @narrow_bswap_overshift2(i96 %x) {
 ; CHECK-LABEL: @narrow_bswap_overshift2(
-; CHECK-NEXT:    [[Z:%.*]] = zext i96 [[X:%.*]] to i128
-; CHECK-NEXT:    [[B:%.*]] = call i128 @llvm.bswap.i128(i128 [[Z]])
-; CHECK-NEXT:    [[S:%.*]] = lshr i128 [[B]], 61
+; CHECK-NEXT:    [[TMP1:%.*]] = call i96 @llvm.bswap.i96(i96 [[X:%.*]])
+; CHECK-NEXT:    [[TMP2:%.*]] = lshr i96 [[TMP1]], 29
+; CHECK-NEXT:    [[S:%.*]] = zext i96 [[TMP2]] to i128
 ; CHECK-NEXT:    ret i128 [[S]]
 ;
   %z = zext i96 %x to i128
@@ -920,6 +921,8 @@ define i128 @narrow_bswap_overshift2(i96 %x) {
   ret i128 %s
 }
 
+; negative test - can't make a bswap with an odd number of bytes
+
 define i32 @not_narrow_bswap(i24 %x) {
 ; CHECK-LABEL: @not_narrow_bswap(
 ; CHECK-NEXT:    [[Z:%.*]] = zext i24 [[X:%.*]] to i32