[ValueTracking] Let ComputeKnownSignBits handle (shl (zext X), C)

Add simple support for looking through a zext when doing
ComputeKnownSignBits for shl. This is valid for the case when
all extended bits are shifted out, because then the number of sign
bits can be found by analysing the zext operand.

The solution here is simple as it only handle a single zext (not
passing remaining left shift amount during recursion). It could be
possible to generalize this in the future by for example passing an
'OffsetFromMSB' parameter to ComputeNumSignBitsImpl, telling it to
calculate number of sign bits starting at some offset from the most
significant bit.

Author: bjope

llvm/lib/Analysis/ValueTracking.cpp

@@ -3757,11 +3757,21 @@ static unsigned ComputeNumSignBitsImpl(const Value *V,
     }
     case Instruction::Shl: {
       const APInt *ShAmt;
+      Value *X = nullptr;
       if (match(U->getOperand(1), m_APInt(ShAmt))) {
         // shl destroys sign bits.
-        Tmp = ComputeNumSignBits(U->getOperand(0), Depth + 1, Q);
-        if (ShAmt->uge(TyBits) ||   // Bad shift.
-            ShAmt->uge(Tmp)) break; // Shifted all sign bits out.
+        if (ShAmt->uge(TyBits))
+          break; // Bad shift.
+        // We can look through a zext (more or less treating it as a sext) if
+        // all extended bits are shifted out.
+        if (match(U->getOperand(0), m_ZExt(m_Value(X))) &&
+            ShAmt->uge(TyBits - X->getType()->getScalarSizeInBits())) {
+          Tmp = ComputeNumSignBits(X, Depth + 1, Q);
+          Tmp += TyBits - X->getType()->getScalarSizeInBits();
+        } else
+          Tmp = ComputeNumSignBits(U->getOperand(0), Depth + 1, Q);
+        if (ShAmt->uge(Tmp))
+          break; // Shifted all sign bits out.
         Tmp2 = ShAmt->getZExtValue();
         return Tmp - Tmp2;
       }

llvm/unittests/Analysis/ValueTrackingTest.cpp

@@ -697,6 +697,36 @@ TEST_F(ValueTrackingTest, ComputeNumSignBits_PR32045) {
   EXPECT_EQ(ComputeNumSignBits(A, M->getDataLayout()), 32u);
 }
 
+TEST_F(ValueTrackingTest, ComputeNumSignBits_shl_ext1) {
+  parseAssembly("define i32 @test(i8 %a) {\n"
+                "  %b = ashr i8 %a, 4\n"
+                "  %c = zext i8 %b to i32\n"
+                "  %A = shl i32 %c, 24\n"
+                "  ret i32 %A\n"
+                "}\n");
+  EXPECT_EQ(ComputeNumSignBits(A, M->getDataLayout()), 5u);
+}
+
+TEST_F(ValueTrackingTest, ComputeNumSignBits_shl_ext2) {
+  parseAssembly("define i32 @test(i8 %a) {\n"
+                "  %b = ashr i8 %a, 4\n"
+                "  %c = zext i8 %b to i32\n"
+                "  %A = shl i32 %c, 26\n"
+                "  ret i32 %A\n"
+                "}\n");
+  EXPECT_EQ(ComputeNumSignBits(A, M->getDataLayout()), 3u);
+}
+
+TEST_F(ValueTrackingTest, ComputeNumSignBits_shl_ext3) {
+  parseAssembly("define i32 @test(i8 %a) {\n"
+                "  %b = ashr i8 %a, 4\n"
+                "  %c = zext i8 %b to i32\n"
+                "  %A = shl i32 %c, 30\n"
+                "  ret i32 %A\n"
+                "}\n");
+  EXPECT_EQ(ComputeNumSignBits(A, M->getDataLayout()), 1u);
+}
+
 // No guarantees for canonical IR in this analysis, so this just bails out.
 TEST_F(ValueTrackingTest, ComputeNumSignBits_Shuffle) {
   parseAssembly(