[PATCH] DataFlowSanitizer; LLVM changes.

[Peter Collingbourne]

================
Comment at: lib/Transforms/Instrumentation/DataFlowSanitizer.cpp:656
@@ +655,3 @@
+         Ofs += 64 / DFS.ShadowWidth) {
+      BasicBlock *NextBB = BasicBlock::Create(*DFS.Ctx, "", F);
+      IRBuilder<> NextIRB(NextBB);
----------------
Evgeniy Stepanov wrote:
> You creating are a lot of basic blocks here. Would it be better to make a loop here an leave it to the loop unroller to make the decision?
> On the other hand, in practice Size must be very limited.
Exactly.  I expect generally about 1 iteration max through this loop.  It was easier to code this without a loop and I don't think it's worth trying to optimise for the uncommon case where the load is large.

================
Comment at: lib/Transforms/Instrumentation/DataFlowSanitizer.cpp:723
@@ +722,3 @@
+  uint64_t Offset = 0;
+  if (Size >= ShadowVecSize) {
+    VectorType *ShadowVecTy = VectorType::get(DFS.ShadowTy, ShadowVecSize);
----------------
Evgeniy Stepanov wrote:
> This looks like an optimization that belongs elsewhere. I wonder if there is a pass in LLVM that will do it for you?
> Also, why 128 bit code here, and only 64 bit in LoadShadow?
> This looks like an optimization that belongs elsewhere. I wonder if there is a pass in LLVM that will do it for you?

I don't think there is one.  I tried optimising the following code with -O3; no improvement:

void f(short *p, short s) {
  for (unsigned i = 8; i != 0; --i, ++p) {
    *p = s;
  }
}

> Also, why 128 bit code here, and only 64 bit in LoadShadow?

Because LoadShadow needs to be able to do equality comparisons and collapse the result into a single bit.  It also needs to be able to compare every 16-bit element of the 128-bit vector against one another. There are some short instruction sequences for the former (on x86), but LLVM doesn't seem to be able to generate them, and I haven't thought too much about the latter yet.


http://llvm-reviews.chandlerc.com/D965