LowerBitSets: Avoid reusing byte set addresses.

[pcc]

Summary

Each use of the byte array uses a different alias. This makes the
backend less likely to reuse previously computed byte array addresses,
improving the security of the CFI mechanism based on this pass.

[jfb]

IIUC this pass prevents redundant load elimination by creating aliases that the compiler doesn't merge?

What's the effect on code size?

Should this be an option, so we can turn it off to compare?

[kcc]

Should this be an option, so we can turn it off to compare?

Yes, please.
The more such options we have the better.
This one could be on by default.

[pcc]

• Add a flag

[jfb]

lgtm

lib/Transforms/IPO/LowerBitSets.cpp

if (BSI.BitSize <= 64) {
     Constant *BitsConst = ConstantInt::get(BitsTy, Bits);
     return createMaskedBitTest(B, BitsConst, BitOffset);
   } else {
     if (!BAI) {
       ++NumByteArraysCreated;
       BAI = createByteArray(BSI);
     }
 
-    Value *ByteAddr = B.CreateGEP(BAI->ByteArray, BitOffset);
+    Constant *ByteArray = BAI->ByteArray;
+    if (!LinkerSubsectionsViaSymbols) {
+      // Each use of the byte array uses a different alias. This makes the
+      // backend less likely to reuse previously computed byte array addresses,
+      // improving the security of the CFI mechanism based on this pass.
+      ByteArray = GlobalAlias::create(
+          BAI->ByteArray->getType()->getElementType(), 0,
+          GlobalValue::PrivateLinkage, "bits_use", ByteArray, M);
+    }
+
+    Value *ByteAddr = B.CreateGEP(ByteArray, BitOffset);
     Value *Byte = B.CreateLoad(ByteAddr);
 
     Value *ByteAndMask = B.CreateAnd(Byte, BAI->Mask);
     return B.CreateICmpNE(ByteAndMask, ConstantInt::get(Int8Ty, 0));
   }
 }
 
 /// Lower a llvm.bitset.test call to its implementation. Returns the value to

test/Transforms/LowerBitSets/simple.ll

 ; CHECK-NODISCARD-DAG: !{!"bitset3", i32* @c, i32 0}
 
 ; Entries whose second operand is null (the result of a global being DCE'd)
 ; should be ignored.
 !7 = !{!"bitset2", null, i32 0}
 
 !llvm.bitsets = !{ !0, !1, !2, !3, !4, !5, !6, !7 }
 
+; CHECK: @bits_use{{[0-9]*}} = private alias i8* @bits{{[0-9]*}}
+; CHECK: @bits_use{{[0-9]*}} = private alias i8* @bits{{[0-9]*}}
+; CHECK: @bits_use{{[0-9]*}} = private alias i8* @bits{{[0-9]*}}
+
 ; CHECK: @a = alias getelementptr inbounds ({ i32, [0 x i8], [63 x i32], [4 x i8], i32, [0 x i8], [2 x i32] }, { i32, [0 x i8], [63 x i32], [4 x i8], i32, [0 x i8], [2 x i32] }* [[G]], i32 0, i32 0)
 ; CHECK: @b = alias getelementptr inbounds ({ i32, [0 x i8], [63 x i32], [4 x i8], i32, [0 x i8], [2 x i32] }, { i32, [0 x i8], [63 x i32], [4 x i8], i32, [0 x i8], [2 x i32] }* [[G]], i32 0, i32 2)
 ; CHECK: @c = alias getelementptr inbounds ({ i32, [0 x i8], [63 x i32], [4 x i8], i32, [0 x i8], [2 x i32] }, { i32, [0 x i8], [63 x i32], [4 x i8], i32, [0 x i8], [2 x i32] }* [[G]], i32 0, i32 4)
 ; CHECK: @d = alias getelementptr inbounds ({ i32, [0 x i8], [63 x i32], [4 x i8], i32, [0 x i8], [2 x i32] }, { i32, [0 x i8], [63 x i32], [4 x i8], i32, [0 x i8], [2 x i32] }* [[G]], i32 0, i32 6)
 
 ; CHECK-DARWIN: @aptr = constant i32* getelementptr inbounds ({ i32, [0 x i8], [63 x i32], [4 x i8], i32, [0 x i8], [2 x i32] }, { i32, [0 x i8], [63 x i32], [4 x i8], i32, [0 x i8], [2 x i32] }* [[G:@[^ ]*]], i32 0, i32 0)
 @aptr = constant i32* @a
 
 ; CHECK-DARWIN: @bptr = constant [63 x i32]* getelementptr inbounds ({ i32, [0 x i8], [63 x i32], [4 x i8], i32, [0 x i8], [2 x i32] }, { i32, [0 x i8], [63 x i32], [4 x i8], i32, [0 x i8], [2 x i32] }* [[G]], i32 0, i32 2)
 @bptr = constant [63 x i32]* @b
 
 ; CHECK-DARWIN: @cptr = constant i32* getelementptr inbounds ({ i32, [0 x i8], [63 x i32], [4 x i8], i32, [0 x i8], [2 x i32] }, { i32, [0 x i8], [63 x i32], [4 x i8], i32, [0 x i8], [2 x i32] }* [[G]], i32 0, i32 4)
 @cptr = constant i32* @c
 
 ; CHECK-DARWIN: @dptr = constant [2 x i32]* getelementptr inbounds ({ i32, [0 x i8], [63 x i32], [4 x i8], i32, [0 x i8], [2 x i32] }, { i32, [0 x i8], [63 x i32], [4 x i8], i32, [0 x i8], [2 x i32] }* [[G]], i32 0, i32 6)
 @dptr = constant [2 x i32]* @d
 
 ; CHECK-DARWIN: [[G]] = private constant
 
-; CHECK: @bits = private alias getelementptr inbounds ([68 x i8], [68 x i8]* [[BA]], i32 0, i32 0)
-; CHECK: @bits1 = private alias getelementptr inbounds ([68 x i8], [68 x i8]* [[BA]], i32 0, i32 0)
+; CHECK: @bits{{[0-9]*}} = private alias getelementptr inbounds ([68 x i8], [68 x i8]* [[BA]], i32 0, i32 0)
+; CHECK: @bits{{[0-9]*}} = private alias getelementptr inbounds ([68 x i8], [68 x i8]* [[BA]], i32 0, i32 0)
 
 declare i1 @llvm.bitset.test(i8* %ptr, metadata %bitset) nounwind readnone
 
 ; CHECK: @foo(i32* [[A0:%[^ ]*]])
 define i1 @foo(i32* %p) {
   ; CHECK-NOT: llvm.bitset.test
 
   ; CHECK: [[R0:%[^ ]*]] = bitcast i32* [[A0]] to i8*
   %pi8 = bitcast i32* %p to i8*
   ; CHECK: [[R1:%[^ ]*]] = ptrtoint i8* [[R0]] to i32
   ; CHECK: [[R2:%[^ ]*]] = sub i32 [[R1]], ptrtoint ({ i32, [0 x i8], [63 x i32], [4 x i8], i32, [0 x i8], [2 x i32] }* [[G]] to i32)
   ; CHECK: [[R3:%[^ ]*]] = lshr i32 [[R2]], 2
   ; CHECK: [[R4:%[^ ]*]] = shl i32 [[R2]], 30
   ; CHECK: [[R5:%[^ ]*]] = or i32 [[R3]], [[R4]]
   ; CHECK: [[R6:%[^ ]*]] = icmp ult i32 [[R5]], 68
   ; CHECK: br i1 [[R6]]
 
-  ; CHECK: [[R8:%[^ ]*]] = getelementptr i8, i8* @bits, i32 [[R5]]
+  ; CHECK: [[R8:%[^ ]*]] = getelementptr i8, i8* @bits_use{{[0-9]*}}, i32 [[R5]]
   ; CHECK: [[R9:%[^ ]*]] = load i8, i8* [[R8]]
   ; CHECK: [[R10:%[^ ]*]] = and i8 [[R9]], 1
   ; CHECK: [[R11:%[^ ]*]] = icmp ne i8 [[R10]], 0
 
   ; CHECK: [[R16:%[^ ]*]] = phi i1 [ false, {{%[^ ]*}} ], [ [[R11]], {{%[^ ]*}} ]
   %x = call i1 @llvm.bitset.test(i8* %pi8, metadata !"bitset1")
 
   ; CHECK-NOT: llvm.bitset.test
define i1 @baz(i32* %p) {
   ; CHECK: [[T1:%[^ ]*]] = ptrtoint i8* [[T0]] to i32
   ; CHECK: [[T2:%[^ ]*]] = sub i32 [[T1]], ptrtoint ({ i32, [0 x i8], [63 x i32], [4 x i8], i32, [0 x i8], [2 x i32] }* [[G]] to i32)
   ; CHECK: [[T3:%[^ ]*]] = lshr i32 [[T2]], 2
   ; CHECK: [[T4:%[^ ]*]] = shl i32 [[T2]], 30
   ; CHECK: [[T5:%[^ ]*]] = or i32 [[T3]], [[T4]]
   ; CHECK: [[T6:%[^ ]*]] = icmp ult i32 [[T5]], 66
   ; CHECK: br i1 [[T6]]
 
-  ; CHECK: [[T8:%[^ ]*]] = getelementptr i8, i8* @bits1, i32 [[T5]]
+  ; CHECK: [[T8:%[^ ]*]] = getelementptr i8, i8* @bits_use{{[0-9]*}}, i32 [[T5]]
   ; CHECK: [[T9:%[^ ]*]] = load i8, i8* [[T8]]
   ; CHECK: [[T10:%[^ ]*]] = and i8 [[T9]], 2
   ; CHECK: [[T11:%[^ ]*]] = icmp ne i8 [[T10]], 0
 
   ; CHECK: [[T16:%[^ ]*]] = phi i1 [ false, {{%[^ ]*}} ], [ [[T11]], {{%[^ ]*}} ]
   %x = call i1 @llvm.bitset.test(i8* %pi8, metadata !"bitset3")
   ; CHECK: ret i1 [[T16]]
   ret i1 %x
 }
 
 ; CHECK-NOT: !llvm.bitsets