KnownBits: refine KnownBits::srem for high-bits

KnownBits::srem does not correctly set the leader zero-bits, omitting
the fact that LHS may be known-negative or known-non-negative. Fix this.

Alive2 proof: https://alive2.llvm.org/ce/z/WcDkSX

llvm/lib/Support/KnownBits.cpp

@@ -1075,9 +1075,13 @@ KnownBits KnownBits::srem(const KnownBits &LHS, const KnownBits &RHS) {
 
   // The sign bit is the LHS's sign bit, except when the result of the
   // remainder is zero. The magnitude of the result should be less than or
-  // equal to the magnitude of the LHS. Therefore any leading zeros that exist
-  // in the left hand side must also exist in the result.
-  Known.Zero.setHighBits(LHS.countMinLeadingZeros());
+  // equal to the magnitude of either operand.
+  if (LHS.isNegative() && Known.isNonZero())
+    Known.One.setHighBits(
+        std::max(LHS.countMinLeadingOnes(), RHS.countMinLeadingOnes()));
+  else if (LHS.isNonNegative())
+    Known.Zero.setHighBits(
+        std::max(LHS.countMinLeadingZeros(), RHS.countMinLeadingZeros()));
   return Known;
 }
 

llvm/test/Analysis/ValueTracking/knownbits-rem.ll

@@ -93,11 +93,7 @@ define i8 @srem_low_bits_know(i8 %xx, i8 %yy) {
 
 define i8 @srem_high_bits_know(i8 %xx, i8 %yy) {
 ; CHECK-LABEL: @srem_high_bits_know(
-; CHECK-NEXT:    [[X:%.*]] = or i8 [[XX:%.*]], -2
-; CHECK-NEXT:    [[Y:%.*]] = and i8 [[YY:%.*]], -4
-; CHECK-NEXT:    [[REM:%.*]] = srem i8 [[X]], [[Y]]
-; CHECK-NEXT:    [[R:%.*]] = and i8 [[REM]], 8
-; CHECK-NEXT:    ret i8 [[R]]
+; CHECK-NEXT:    ret i8 8
 ;
   %x = or i8 %xx, -2
   %y = and i8 %yy, -4

llvm/test/CodeGen/ARM/select-imm.ll

@@ -655,14 +655,11 @@ define i1 @t10() {
 ; V8MBASE-NEXT:    .pad #8
 ; V8MBASE-NEXT:    sub sp, #8
 ; V8MBASE-NEXT:    movs r0, #7
-; V8MBASE-NEXT:    mvns r0, r0
-; V8MBASE-NEXT:    str r0, [sp]
-; V8MBASE-NEXT:    adds r1, r0, #5
-; V8MBASE-NEXT:    str r1, [sp, #4]
-; V8MBASE-NEXT:    sdiv r2, r1, r0
-; V8MBASE-NEXT:    muls r2, r0, r2
-; V8MBASE-NEXT:    subs r0, r1, r2
-; V8MBASE-NEXT:    subs r1, r0, r1
+; V8MBASE-NEXT:    mvns r1, r0
+; V8MBASE-NEXT:    str r1, [sp]
+; V8MBASE-NEXT:    adds r0, r1, #5
+; V8MBASE-NEXT:    str r0, [sp, #4]
+; V8MBASE-NEXT:    adds r1, #8
 ; V8MBASE-NEXT:    rsbs r0, r1, #0
 ; V8MBASE-NEXT:    adcs r0, r1
 ; V8MBASE-NEXT:    add sp, #8
@@ -719,7 +716,7 @@ define i1 @t11() {
 ; ARMT2-NEXT:    and r1, r1, r2
 ; ARMT2-NEXT:    orr r0, r1, r0
 ; ARMT2-NEXT:    str r0, [sp]
-; ARMT2-NEXT:    bfc r0, #12, #20
+; ARMT2-NEXT:    and r0, r0, #15
 ; ARMT2-NEXT:    sub r0, r0, #3
 ; ARMT2-NEXT:    clz r0, r0
 ; ARMT2-NEXT:    lsr r0, r0, #5
@@ -781,7 +778,7 @@ define i1 @t11() {
 ; THUMB2-NEXT:    ands r1, r2
 ; THUMB2-NEXT:    orrs r0, r1
 ; THUMB2-NEXT:    str r0, [sp]
-; THUMB2-NEXT:    bfc r0, #12, #20
+; THUMB2-NEXT:    and r0, r0, #15
 ; THUMB2-NEXT:    subs r0, #3
 ; THUMB2-NEXT:    clz r0, r0
 ; THUMB2-NEXT:    lsrs r0, r0, #5