[MemoryLocation] Support memset_chk in getForArgument.

[fhahn]

Summary

The size argument for memset_chk is an upper bound for the size of the
pointer argument. memset_chk may write less than the specified length,
if it exceeds the specified max size and aborts.

[nikic]

LGTM

llvm/lib/Analysis/MemoryLocation.cpp

220

exceed -> exceeded/exceeds?

[fhahn]

llvm/lib/Analysis/MemoryLocation.cpp

220

Thanks, updated to exceeds in the committed version!

llvm/lib/Analysis/MemoryLocation.cpp

MemoryLocation MemoryLocation::getForArgument(const CallBase *Call,
 
   // We can bound the aliasing properties of memset_pattern16 just as we can
   // for memcpy/memset.  This is particularly important because the
   // LoopIdiomRecognizer likes to turn loops into calls to memset_pattern16
   // whenever possible.
   LibFunc F;
   if (TLI && TLI->getLibFunc(*Call, F) && TLI->has(F)) {
     switch (F) {
+    case LibFunc_memset_chk: {
+      assert(ArgIdx == 0 && "Invalid argument index for memset_chk");
+      LocationSize Size = LocationSize::afterPointer();
+      if (const auto *Len = dyn_cast<ConstantInt>(Call->getArgOperand(2))) {
+        // memset_chk writes at most Len bytes. It may write less, if Len
+        // exceeds the specified max size and aborts.
+        Size = LocationSize::upperBound(Len->getZExtValue());
+      }
+      return MemoryLocation(Arg, Size, AATags);
+    }
     case LibFunc_memset_pattern16:
       assert((ArgIdx == 0 || ArgIdx == 1) &&
              "Invalid argument index for memset_pattern16");
       if (ArgIdx == 1)
         return MemoryLocation(Arg, LocationSize::precise(16), AATags);
       if (const ConstantInt *LenCI =
               dyn_cast<ConstantInt>(Call->getArgOperand(2)))
         return MemoryLocation(Arg, LocationSize::precise(LenCI->getZExtValue()),

llvm/test/Analysis/BasicAA/libfuncs.ll

 
 declare i8* @__memset_chk(i8* writeonly, i32, i64, i64)
 
 ; CHECK-LABEL: Function: test_memset_chk_const_size
 define i8* @test_memset_chk_const_size(i8* noalias %a, i64 %n) {
 ; CHECK:       Just Mod (MustAlias):  Ptr: i8* %a <->  %res = tail call i8* @__memset_chk(i8* %a, i32 0, i64 4, i64 %n)
 ; CHECK-NEXT:  Just Mod:  Ptr: i8* %res <->  %res = tail call i8* @__memset_chk(i8* %a, i32 0, i64 4, i64 %n)
 ; CHECK-NEXT:  Just Mod:  Ptr: i8* %a.gep.1 <->  %res = tail call i8* @__memset_chk(i8* %a, i32 0, i64 4, i64 %n)
-; CHECK-NEXT:  Just Mod:  Ptr: i8* %a.gep.5 <->  %res = tail call i8* @__memset_chk(i8* %a, i32 0, i64 4, i64 %n)
+; CHECK-NEXT:  NoModRef:  Ptr: i8* %a.gep.5 <->  %res = tail call i8* @__memset_chk(i8* %a, i32 0, i64 4, i64 %n)
 ;
 entry:
   %res = tail call i8* @__memset_chk(i8* %a, i32 0, i64 4, i64 %n)
   %a.gep.1 = getelementptr i8, i8* %a, i32 1
   store i8 0, i8* %a.gep.1
   %a.gep.5 = getelementptr i8, i8* %a, i32 5
   store i8 1, i8* %a.gep.5
   ret i8* %res