[InstCombine] Optimize (icmp slt (1 << Y), 1) -> (icmp eq Y, BitWidth-1).

[craig.topper]

Summary

The code tried to do this for (icmp sle (1 << Y), 0), but that is
canonicalized to sgt before we get there.

Simplify the code by removing the unreachable SGE and SLE handling.

Also remove the (1 << Y) >=u 2147483648 and (1 << Y) <u 2147483648
handling since those are canonicalized to (1 << Y) <s 0 and
(1 << Y) >=s 0 before we get there.

[spatel]

It's good cleanup and seems correct, but it can be generalized even more (could make it another patch if you prefer):
https://alive2.llvm.org/ce/z/3YB2vs

Need more tests like this:
https://alive2.llvm.org/ce/z/hVNnPJ

[craig.topper]

In D141753#4053721, @spatel wrote:

It's good cleanup and seems correct, but it can be generalized even more (could make it another patch if you prefer):
https://alive2.llvm.org/ce/z/3YB2vs

Need more tests like this:
https://alive2.llvm.org/ce/z/hVNnPJ

I'd prefer to make it another patch. I haven't observed this in the wild. I was looking in the file for something else and just happened to notice the asymmetry. Then I looked at the line coverage report and saw the untested lines.

[spatel]

LGTM - please add a todo comment if you aren’t planning to do the follow up patch immediately.

llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

if (!CIsPowerOf2) {
       // (1 << Y) >= 30 -> Y >  4
       // (1 << Y) >  30 -> Y >  4
       if (Pred == ICmpInst::ICMP_ULT)
         Pred = ICmpInst::ICMP_ULE;
       else if (Pred == ICmpInst::ICMP_UGE)
         Pred = ICmpInst::ICMP_UGT;
     }
 
-    // (1 << Y) >= 2147483648 -> Y >= 31 -> Y == 31
-    // (1 << Y) <  2147483648 -> Y <  31 -> Y != 31
     unsigned CLog2 = C.logBase2();
-    if (CLog2 == TypeBits - 1) {
-      if (Pred == ICmpInst::ICMP_UGE)
-        Pred = ICmpInst::ICMP_EQ;
-      else if (Pred == ICmpInst::ICMP_ULT)
-        Pred = ICmpInst::ICMP_NE;
-    }
     return new ICmpInst(Pred, Y, ConstantInt::get(ShiftType, CLog2));
   } else if (Cmp.isSigned()) {
     Constant *BitWidthMinusOne = ConstantInt::get(ShiftType, TypeBits - 1);
-    if (C.isAllOnes()) {
-      // (1 << Y) <= -1 -> Y == 31
-      if (Pred == ICmpInst::ICMP_SLE)
-        return new ICmpInst(ICmpInst::ICMP_EQ, Y, BitWidthMinusOne);
-
-      // (1 << Y) >  -1 -> Y != 31
-      if (Pred == ICmpInst::ICMP_SGT)
-        return new ICmpInst(ICmpInst::ICMP_NE, Y, BitWidthMinusOne);
-    } else if (!C) {
-      // (1 << Y) <  0 -> Y == 31
-      // (1 << Y) <= 0 -> Y == 31
-      if (Pred == ICmpInst::ICMP_SLT || Pred == ICmpInst::ICMP_SLE)
-        return new ICmpInst(ICmpInst::ICMP_EQ, Y, BitWidthMinusOne);
-
-      // (1 << Y) >= 0 -> Y != 31
-      // (1 << Y) >  0 -> Y != 31
-      if (Pred == ICmpInst::ICMP_SGT || Pred == ICmpInst::ICMP_SGE)
-        return new ICmpInst(ICmpInst::ICMP_NE, Y, BitWidthMinusOne);
-    }
+    // (1 << Y) >  0 -> Y != 31
+    // (1 << Y) > -1 -> Y != 31
+    // TODO: This can be generalized to any negative constant.
+    if (Pred == ICmpInst::ICMP_SGT && (C.isZero() || C.isAllOnes()))
+
+    // (1 << Y) <  1 -> Y == 31
+    // TODO: This can be generalized to any negative constant except signed min.
+    if (Pred == ICmpInst::ICMP_SLT && (C.isZero() || C.isOne()))
   } else if (Cmp.isEquality() && CIsPowerOf2) {
     return new ICmpInst(Pred, Y, ConstantInt::get(ShiftType, C.logBase2()));
   }
 
   return nullptr;
 }
 
 /// Fold icmp (shl X, Y), C.

llvm/test/Transforms/InstCombine/icmp.ll

 ;
   %shl = shl <2 x i32> <i32 1, i32 1>, %V
   %cmp = icmp ult <2 x i32> %shl, <i32 2147483648, i32 2147483648>
   ret <2 x i1> %cmp
 }
 
 define i1 @icmp_shl_1_V_sle_0(i32 %V) {
 ; CHECK-LABEL: @icmp_shl_1_V_sle_0(
-; CHECK-NEXT:    [[SHL:%.*]] = shl nuw i32 1, [[V:%.*]]
-; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i32 [[SHL]], 1
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i32 [[V:%.*]], 31
 ; CHECK-NEXT:    ret i1 [[CMP]]
 ;
   %shl = shl i32 1, %V
   %cmp = icmp sle i32 %shl, 0
   ret i1 %cmp
 }
 
 define <2 x i1> @icmp_shl_1_V_sle_0_vec(<2 x i32> %V) {
 ; CHECK-LABEL: @icmp_shl_1_V_sle_0_vec(
-; CHECK-NEXT:    [[SHL:%.*]] = shl nuw <2 x i32> <i32 1, i32 1>, [[V:%.*]]
-; CHECK-NEXT:    [[CMP:%.*]] = icmp slt <2 x i32> [[SHL]], <i32 1, i32 1>
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq <2 x i32> [[V:%.*]], <i32 31, i32 31>
 ; CHECK-NEXT:    ret <2 x i1> [[CMP]]
 ;
   %shl = shl <2 x i32> <i32 1, i32 1>, %V
   %cmp = icmp sle <2 x i32> %shl, <i32 0, i32 0>
   ret <2 x i1> %cmp
 }
 
 define i1 @icmp_shl_1_V_sgt_0(i32 %V) {