[NFC][CLANG] Fix Static Code Analyzer Concerns with bad bit right shift operation in getNVPTXLaneID()

[Manna]

Summary

In getNVPTXLaneID(CodeGenFunction &), the value of LaneIDBits is 4294967295 since function call llvm::Log2_32(CGF->getTarget()->getGridValue().GV_Warp_Size) might return 4294967295.

unsigned LaneIDBits =
     llvm::Log2_32(CGF.getTarget().getGridValue().GV_Warp_Size);
unsigned LaneIDMask = ~0u >> (32u - LaneIDBits);

The shift amount (32U - LaneIDBits) might be 33, So right shifting by more than 31 bits will have undefined behavior.

This patch adds an assert to guard the LaneIDBits overflow issue for valid LaneIDMask value.

[Manna]

I have updated patch.

[tra]

In practice we're guaranteed by GPU architecture that the warp size will always be small enough to fit in 32 bits.

Also log2_32 will never return a value larger than 32.

Does this assert help with anything else other than potential undefined behavior?

[tahonermann]

The static analysis results look suspect since there does not appear to be a way for Log2_32 to return a value larger than 31. Regardless, the change looks safe to me; GV_Warp_Size should always be greater than 0, so Log2_32 should always return a non-negative number that is less than 32.

[Manna]

Thank you @tahonermann and @tra for reviews and comments!

clang/lib/CodeGen/CGOpenMPRuntimeGPU.cpp

 
 /// Get the id of the current lane in the Warp.
 /// We assume that the warp size is 32, which is always the case
 /// on the NVPTX device, to generate more efficient code.
 static llvm::Value *getNVPTXLaneID(CodeGenFunction &CGF) {
   CGBuilderTy &Bld = CGF.Builder;
   unsigned LaneIDBits =
       llvm::Log2_32(CGF.getTarget().getGridValue().GV_Warp_Size);
+  assert(LaneIDBits < 32 && "Invalid LaneIDBits size in NVPTX device.");
   unsigned LaneIDMask = ~0u >> (32u - LaneIDBits);
   auto &RT = static_cast<CGOpenMPRuntimeGPU &>(CGF.CGM.getOpenMPRuntime());
   return Bld.CreateAnd(RT.getGPUThreadID(CGF), Bld.getInt32(LaneIDMask),
                        "nvptx_lane_id");
 }
 
 CGOpenMPRuntimeGPU::ExecutionMode
 CGOpenMPRuntimeGPU::getExecutionMode() const {