[PR] Stack Instrumentation Fixup

[elazaro-riverside]

This PR fixes inconsistencies with our stack instrumentation approach. During the E-BOSS PI meeting, we observed that the stack instrumentation was inconsistent for the challenge problems supplied by TA2.

The implementation before this PR, tried to pad each LLVM stack allocation (alloca) instruction with an additional byte to act as a guard bit between allocations. There was an effort to implement the padding using a struct with an additional i8 field but LLVM's optimizations would change the layout of the transformed alloca causing the instrumentation to be inconsistent. Changing the form of the alloca is dangerous because LLVM apply optimizations based on the shape and size of it. The solution we arrived at takes the original array type alloca and creates another alloca with [N + 1 x T] elements in it by creating a new alloca with a new shape. Getting this right was tricky but now it works. In the non-constant size case, we need to insert an add instruction after the size argument is zero-extended but before the allocation occurs.

We also realized in the challenge problems that Clang lowers VLA's (variable length arrays) into alloca's of a single type and a non-constant argument. This also caused issues for the instrumentation because we had not considered handling VLA's which live on the stack but have dynamic sizing.

An additional consideration, it is possible that an alloca is not executed along a particular control flow path even though it appears in the IR. To address this, Jackson proposed a per function pointer variable (shadow slot) initialized to null. After each instrumented alloca, the instrumentation inserts a store instruction to store the stack address. At the function epilogue, the instrumentation inserts load instruction to load the pointer from memory and pass it as an argument to __resolve_invalidate_stack to invalidate the associated stack region. If the variable remains null the call becomes a no-op, ensuring that no invalid stack deallocation occurs for alloca's that were never executed along the taken control flow path.