[CHERIoT] Fix AllNonPositive condition in the Cheriot non-reassociation path in LICM.

The previous implementation was incorrectly too permissive, allowing hoisting that should have been disallowed.

Author: resistor

llvm/lib/Transforms/Scalar/LICM.cpp

@@ -2546,7 +2546,9 @@ static bool hoistGEP(Instruction &I, Loop &L, ICFLoopSafetyInfo &SafetyInfo,
                           (ABI == "cheriot" || ABI == "cheriot-baremetal");
   if (MustStayInBounds) {
     auto NonPositive = [&](Value *V) {
-      return !isKnownPositive(V, SimplifyQuery(DL, DT, AC, GEP));
+      if (const auto *Cst = dyn_cast<Constant>(V))
+        return Cst->isNullValue();
+      return isKnownNegative(V, SimplifyQuery(DL, DT, AC, GEP));
     };
     bool AllNonNeg = all_of(Src->indices(), NonNegative) &&
                      all_of(GEP->indices(), NonNegative);

llvm/test/Transforms/LICM/cheriot-gep-reorder.ll

@@ -56,8 +56,48 @@ do.end:
   ret void
 }
 
+; CHECK-LABEL: @zot
+; CHECK: bb:
+; CHECK-NOT: getelementptr
+; CHECK: bb12:
+; CHECK: getelementptr
+; CHECK: getelementptr
+; CHECK: call void @bar
+
+define void @zot(ptr addrspace(200) %arg) {
+bb:
+  %alloca = alloca [25 x i64], align 8, addrspace(200)
+  br label %bb8
+
+bb8:
+  %phi = phi i32 [ 0, %bb ], [ %add16, %bb14 ]
+  %phi9 = phi i32 [ 0, %bb ], [ %select, %bb14 ]
+  %icmp = icmp ult i32 %phi9, 256
+  br i1 %icmp, label %bb10, label %bb17
+
+bb10:
+  %icmp11 = icmp eq i32 %phi, 4
+  br i1 %icmp11, label %bb12, label %bb14
+
+bb12:
+  %getelementptr = getelementptr inbounds nuw i16, ptr addrspace(200) %arg, i32 %phi9
+  %getelementptr13 = getelementptr inbounds i8, ptr addrspace(200) %getelementptr, i32 -6
+  call void @bar(ptr addrspace(200) %getelementptr13)
+  br label %bb14
+
+bb14:
+  %icmp15 = icmp ne i32 %phi9, 255
+  %add = add nuw nsw i32 %phi9, 2
+  %zext = zext i1 %icmp15 to i32
+  %add16 = add i32 %zext, %phi
+  %select = select i1 %icmp15, i32 %add, i32 256
+  br label %bb8
+
+bb17:
+  ret void
+}
 
-declare void @bar(ptr addrspace(200) noundef)
+declare void @bar(ptr addrspace(200))
 
 !llvm.module.flags = !{!0}
 !0 = !{i32 1, !"target-abi", !"cheriot"}