[PATCH 1/2] Add a "probe-stack" attribute

[Zoxc]

Summary

This attribute is used to ensure the guard page is triggered on stack
overflow. Stack frames larger than the guard page size will generate a
call to __probestack to touch each page so the guard page won't be skipped.

[majnemer]

The MachineFunction changes should be in D9654. The inliner changes need a testcase.

include/llvm/CodeGen/MachineFunction.h

286

Please make this function const.

[Zoxc]

Added the test

[majnemer]

lib/Transforms/IPO/Inliner.cpp

138

Any reason why this can't just be Caller->addFnAttr("probe-stack"); ?

[Zoxc]

No.

[Zoxc]

majnemer: Do you have any objections to merging this?

[majnemer]

In D9653#206756, @Zoxc wrote:

majnemer: Do you have any objections to merging this?

I'd be far more comfortable if probe-stack took an argument instead of hard coding the name a specific stack probing function.

[Zoxc]

"probe-stack" is only intended to guarantee that a guard page can be triggered, not to dictate how. __probestack is intended to be added to compiler-rt as to aid implementation (and Windows toolchains have existing functions for this). Ideally LLVM would inline it (as GCC does), but that would require more invasive changes to X86FrameLowering (emitPrologue does not seem like the ideal place to emit a loop).

[Zoxc]

Added documentation.

[Zoxc]

Rebased patch

[whitequark]

@majnemer ping? I am also interested in this.

include/llvm/CodeGen/MachineFunction.h

   /// Should we be emitting segmented stack stuff for the function
   bool shouldSplitStack();
 
+  /// Should we be probing the stack for the function.
+  /// Probing the stack means that we must read or write to the stack on every
+  /// page. This is to ensure that a guard page will be hit and stack overflow
+  /// can be detected. We insert instructions to do this when allocating from
+  /// the stack.
+  bool shouldProbeStack();
+
   /// getNumBlockIDs - Return the number of MBB ID's allocated.
   ///
   unsigned getNumBlockIDs() const { return (unsigned)MBBNumbering.size(); }

lib/CodeGen/MachineFunction.cpp

   return getFunction()->hasFnAttribute("split-stack");
 }
 
+/// Should we be probing the stack for the function.
+/// Probing the stack means that we must read or write to the stack on every
+/// page. This is to ensure that a guard page will be hit and stack overflow
+/// can be detected. We insert instructions to do this when allocating from
+/// the stack.
+bool MachineFunction::shouldProbeStack() {
+  return getFunction()->hasFnAttribute("probe-stack");
+}
+
 /// RenumberBlocks - This discards all of the MachineBasicBlock numbers and
 /// recomputes them.  This guarantees that the MBB numbers are sequential,
 /// dense, and match the ordering of the blocks within the function.  If a

lib/Transforms/IPO/Inliner.cpp

 
   AdjustCallerSSPLevel(Caller, Callee);
 
+  // If the callee requires stack probes, we ensure that the caller will
+  // require those too
+  if (Callee->hasFnAttribute("probe-stack")) {
+    Caller->addFnAttr("probe-stack", "");
+  }
+
   // Look at all of the allocas that we inlined through this call site.  If we
   // have already inlined other allocas through other calls into this function,
   // then we know that they have disjoint lifetimes and that we can merge them.