[ValueTracking] Implied conditions for lshr

[caojoshua]

Summary

V1 >> V2 u<= V1 for any V1, V2

This works for lshr and any div's that are changed to lshr's

This fixes issues in clang and rustc:
https://github.com/llvm/llvm-project/issues/62441
https://github.com/rust-lang/rust/issues/110971

[caojoshua]

I've confirmed this fixes the clang issue. Not sure how to test for rustc, but it should fix based on what I'm seeing in the IR.

[caojoshua]

ping

[goldstein.w.n]

LGTM.

llvm/lib/Analysis/ValueTracking.cpp

static bool isTruePredicate(CmpInst::Predicate Pred, const Value *LHS,
 
   case CmpInst::ICMP_ULE: {
     const APInt *C;
 
     // LHS u<= LHS +_{nuw} C   for any C
     if (match(RHS, m_NUWAdd(m_Specific(LHS), m_APInt(C))))
       return true;
 
+    // RHS >> V u<= RHS for any V
+    if (match(LHS, m_LShr(m_Specific(RHS), m_Value())))
+      return true;
+
     // Match A to (X +_{nuw} CA) and B to (X +_{nuw} CB)
     auto MatchNUWAddsToSameValue = [&](const Value *A, const Value *B,
                                        const Value *&X,
                                        const APInt *&CA, const APInt *&CB) {
       if (match(A, m_NUWAdd(m_Value(X), m_APInt(CA))) &&
           match(B, m_NUWAdd(m_Specific(X), m_APInt(CB))))
         return true;
 

llvm/test/Transforms/InstSimplify/implies.ll

;
   ret i1 %res
 }
 
 ; (X >> Z) <=(u) Y ==> X <=(u) Y
 define i1 @test_shift(i64 %x, i64 %y, i64 %shift) {
 ; CHECK-LABEL: @test_shift(
 ; CHECK-NEXT:    [[LSHR:%.*]] = lshr i64 [[X:%.*]], [[SHIFT:%.*]]
 ; CHECK-NEXT:    [[ICMP1:%.*]] = icmp ugt i64 [[LSHR]], [[Y:%.*]]
-; CHECK-NEXT:    [[ICMP2:%.*]] = icmp ugt i64 [[X]], [[Y]]
 ; CHECK-NEXT:    call void @llvm.assume(i1 [[ICMP1]])
-; CHECK-NEXT:    ret i1 [[ICMP2]]
+; CHECK-NEXT:    ret i1 true
 ;
   %lshr = lshr i64 %x, %shift
   %icmp1 = icmp ugt i64 %lshr, %y
   %icmp2 = icmp ugt i64 %x, %y
   call void @llvm.assume(i1 %icmp1)
   ret i1 %icmp2
 }
 
;
   tail call void @llvm.assume(i1 %2)
   %3 = icmp eq i64 %x, %y
   ret i1 %3
 }
 
 ; i <u L ==> i >> C <u L
 define i1 @lshr_constant(i32 %length.i, i32 %i) {
 ; CHECK-LABEL: @lshr_constant(
-; CHECK-NEXT:    [[SHL:%.*]] = lshr i32 [[I:%.*]], 1
-; CHECK-NEXT:    [[VAR29:%.*]] = icmp ult i32 [[I]], [[LENGTH_I:%.*]]
-; CHECK-NEXT:    [[VAR30:%.*]] = icmp ult i32 [[SHL]], [[LENGTH_I]]
-; CHECK-NEXT:    [[RES:%.*]] = icmp ule i1 [[VAR29]], [[VAR30]]
-; CHECK-NEXT:    ret i1 [[RES]]
+; CHECK-NEXT:    ret i1 true
 ;
   %shl = lshr i32 %i, 1
   %var29 = icmp ult i32 %i, %length.i
   %var30 = icmp ult i32 %shl, %length.i
   %res = icmp ule i1 %var29, %var30
   ret i1 %res
 }
 
 ; i <u L ==> i >> V <u L
 define i1 @lshr_value(i32 %length.i, i32 %i, i32 %v) {
 ; CHECK-LABEL: @lshr_value(
-; CHECK-NEXT:    [[SHL:%.*]] = lshr i32 [[I:%.*]], [[V:%.*]]
-; CHECK-NEXT:    [[VAR29:%.*]] = icmp ult i32 [[I]], [[LENGTH_I:%.*]]
-; CHECK-NEXT:    [[VAR30:%.*]] = icmp ult i32 [[SHL]], [[LENGTH_I]]
-; CHECK-NEXT:    [[RES:%.*]] = icmp ule i1 [[VAR29]], [[VAR30]]
-; CHECK-NEXT:    ret i1 [[RES]]
+; CHECK-NEXT:    ret i1 true
 ;
   %shl = lshr i32 %i, %v
   %var29 = icmp ult i32 %i, %length.i
   %var30 = icmp ult i32 %shl, %length.i
   %res = icmp ule i1 %var29, %var30
   ret i1 %res
 }
 
 declare void @llvm.assume(i1)