[Analysis] Support bitreverse in -demanded-bits pass

[modocache]

Summary

• Add a bitreverse case in the demanded bits analysis pass.
• Add tests for the bitreverse (and bswap) intrinsic in the demanded bits pass.
• Add a test case to the BDCE tests: that manipulations to high-order bits are eliminated once the bits are reversed and then right-shifted.

[jmolloy]

LGTM, thanks!

[modocache]

Great, thanks for the review, @jmolloy!

lib/Analysis/DemandedBits.cpp

case Instruction::Invoke:
     if (const IntrinsicInst *II = dyn_cast<IntrinsicInst>(UserI))
       switch (II->getIntrinsicID()) {
       default: break;
       case Intrinsic::bswap:
         // The alive bits of the input are the swapped alive bits of
         // the output.
         AB = AOut.byteSwap();
         break;
+      case Intrinsic::bitreverse:
+        AB = AOut.reverseBits();
+        break;
       case Intrinsic::ctlz:
         if (OperandNo == 0) {
           // We need some output bits, so we need all bits of the
           // input to the left of, and including, the leftmost bit
           // known to be one.
           ComputeKnownBits(BitWidth, I, nullptr);
           AB = APInt::getHighBitsSet(BitWidth,
                  std::min(BitWidth, KnownOne.countLeadingZeros()+1));

test/Analysis/DemandedBits/intrinsics.ll

+; RUN: opt -S -demanded-bits -analyze < %s | FileCheck %s
+; RUN: opt -S -disable-output -passes="print<demanded-bits>" < %s 2>&1 | FileCheck %s
+
+; CHECK-DAG: DemandedBits: 0xFF000000 for   %1 = or i32 %x, 1
+; CHECK-DAG: DemandedBits: 0xFF for   %2 = call i32 @llvm.bitreverse.i32(i32 %1)
+; CHECK-DAG: DemandedBits: 0xFF for   %3 = trunc i32 %2 to i8
+define i8 @test_bswap(i32 %x) {
+  %1 = or i32 %x, 1
+  %2 = call i32 @llvm.bswap.i32(i32 %1)
+  %3 = trunc i32 %2 to i8
+  ret i8 %3
+}
+declare i32 @llvm.bswap.i32(i32)
+
+; CHECK-DAG: DemandedBits: 0xFF000000 for   %1 = or i32 %x, 1
+; CHECK-DAG: DemandedBits: 0xFF for   %2 = call i32 @llvm.bswap.i32(i32 %1)
+; CHECK-DAG: DemandedBits: 0xFF for   %3 = trunc i32 %2 to i8
+define i8 @test_bitreverse(i32 %x) {
+  %1 = or i32 %x, 1
+  %2 = call i32 @llvm.bitreverse.i32(i32 %1)
+  %3 = trunc i32 %2 to i8
+  ret i8 %3
+}
+declare i32 @llvm.bitreverse.i32(i32)
+

test/Transforms/BDCE/basic.ll

 ; CHECK: tail call signext i32 @foo(i32 signext 4)
 ; CHECK: ret i32
 }
 
 ; Function Attrs: nounwind readnone
 declare i32 @llvm.bswap.i32(i32) #0
 
 ; Function Attrs: nounwind readnone
+define signext i32 @tim(i32 signext %x) #0 {
+entry:
+  %call = tail call signext i32 @foo(i32 signext 5) #0
+  %and = and i32 %call, 536870912
+  %or = or i32 %and, %x
+  %call1 = tail call signext i32 @foo(i32 signext 3) #0
+  %and2 = and i32 %call1, 1073741824
+  %or3 = or i32 %or, %and2
+  %call4 = tail call signext i32 @foo(i32 signext 2) #0
+  %and5 = and i32 %call4, 16
+  %or6 = or i32 %or3, %and5
+  %call7 = tail call signext i32 @foo(i32 signext 1) #0
+  %and8 = and i32 %call7, 32
+  %or9 = or i32 %or6, %and8
+  %call10 = tail call signext i32 @foo(i32 signext 0) #0
+  %and11 = and i32 %call10, 64
+  %or12 = or i32 %or9, %and11
+  %call13 = tail call signext i32 @foo(i32 signext 4) #0
+  %and14 = and i32 %call13, 128
+  %or15 = or i32 %or12, %and14
+  %bs = tail call i32 @llvm.bitreverse.i32(i32 %or15) #0
+  %shr = ashr i32 %bs, 4
+  ret i32 %shr
+
+; CHECK-LABEL: @tim
+; CHECK-NOT: tail call signext i32 @foo(i32 signext 5)
+; CHECK-NOT: tail call signext i32 @foo(i32 signext 3)
+; CHECK: tail call signext i32 @foo(i32 signext 2)
+; CHECK: tail call signext i32 @foo(i32 signext 1)
+; CHECK: tail call signext i32 @foo(i32 signext 0)
+; CHECK: tail call signext i32 @foo(i32 signext 4)
+; CHECK: ret i32
+}
+
+; Function Attrs: nounwind readnone
+declare i32 @llvm.bitreverse.i32(i32) #0
+
+; Function Attrs: nounwind readnone
 define signext i32 @tar2(i32 signext %x) #0 {
 entry:
   %call = tail call signext i32 @foo(i32 signext 5) #0
   %and = and i32 %call, 33554432
   %or = or i32 %and, %x
   %call1 = tail call signext i32 @foo(i32 signext 3) #0
   %and2 = and i32 %call1, 67108864
   %or3 = or i32 %or, %and2