[clang] [llvm] [RISCV] Add stack clash protection (PR #117612)

[Craig Topper via cfe-commits]

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@@ -26,6 +27,35 @@ MachineFunctionInfo *RISCVMachineFunctionInfo::clone(
   return DestMF.cloneInfo<RISCVMachineFunctionInfo>(*this);
 }
 
+RISCVMachineFunctionInfo::RISCVMachineFunctionInfo(const Function &F,
+                                                   const RISCVSubtarget *STI) {
+
+  // The default stack probe size is 4096 if the function has no
+  // stack-probe-size attribute. This is a safe default because it is the
+  // smallest possible guard page size.
+  uint64_t ProbeSize = 4096;
+  if (F.hasFnAttribute("stack-probe-size"))
+    ProbeSize = F.getFnAttributeAsParsedInteger("stack-probe-size");
+  else if (const auto *PS = mdconst::extract_or_null<ConstantInt>(
+               F.getParent()->getModuleFlag("stack-probe-size")))
+    ProbeSize = PS->getZExtValue();
+  assert(int64_t(ProbeSize) > 0 && "Invalid stack probe size");
+
+  // Round down to the stack alignment.
+  uint64_t StackAlign =
+      STI->getFrameLowering()->getTransientStackAlign().value();
+  ProbeSize = std::max(StackAlign, ProbeSize & ~(StackAlign - 1U));
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topperc wrote:

Can we use `std::max(StackAlign, alignDown(ProbeSize, StackAlign))`?

https://github.com/llvm/llvm-project/pull/117612