[clang-cl] Support the /HOTPATCH flag

This patch adds support for the MSVC /HOTPATCH flag: https://docs.microsoft.com/sv-se/cpp/build/reference/hotpatch-create-hotpatchable-image?view=msvc-170&viewFallbackFrom=vs-2019

The flag is translated to a new -fms-hotpatch flag, which in turn adds a 'patchable-function' attribute for each function in the TU. This is then picked up by the PatchableFunction pass which would generate a TargetOpcode::PATCHABLE_OP of minsize = 2 (which means the target instruction must resolve to at least two bytes). TargetOpcode::PATCHABLE_OP is only implemented for x86/x64. When targetting ARM/ARM64, /HOTPATCH isn't required (instructions are always 2/4 bytes and suitable for hotpatching).

Additionally, when using /Z7, we generate a 'hot patchable' flag in the CodeView debug stream, in the S_COMPILE3 record. This flag is then picked up by LLD (or link.exe) and is used in conjunction with the linker /FUNCTIONPADMIN flag to generate extra space before each function, to accommodate for live patching long jumps. Please see: https://github.com/llvm/llvm-project/blob/d703b922961e0d02a5effdd4bfbb23ad50a3cc9f/lld/COFF/Writer.cpp#L1298

The outcome is that we can finally use Live++ or Recode along with clang-cl.

NOTE: It seems that MSVC cl.exe always enables /HOTPATCH on x64 by default, although if we did the same I thought we might generate sub-optimal code (if this flag was active by default). Additionally, MSVC always generates a .debug$S section and a S_COMPILE3 record, which Clang doesn't do without /Z7. Therefore, the following MSVC command-line "cl /c file.cpp" would have to be written with Clang such as "clang-cl /c file.cpp /HOTPATCH /Z7" in order to obtain the same result.

Depends on D43002, D80833 and D81301 for the full feature.

Differential Revision: https://reviews.llvm.org/D116511

1 files changed, 7 insertions, 0 deletions

diff --git a/clang/lib/CodeGen/CodeGenFunction.cpp b/clang/lib/CodeGen/CodeGenFunction.cpp
index 54ddbff..50e1638 100644
--- a/clang/lib/CodeGen/CodeGenFunction.cpp
+++ b/clang/lib/CodeGen/CodeGenFunction.cpp
@@ -882,6 +882,13 @@ void CodeGenFunction::StartFunction(GlobalDecl GD, QualType RetTy,
     if (Offset)
       Fn->addFnAttr("patchable-function-prefix", std::to_string(Offset));
   }
+  // Instruct that functions for COFF/CodeView targets should start with a
+  // patchable instruction, but only on x86/x64. Don't forward this to ARM/ARM64
+  // backends as they don't need it -- instructions on these architectures are
+  // always atomically patchable at runtime.
+  if (CGM.getCodeGenOpts().HotPatch &&
+      getContext().getTargetInfo().getTriple().isX86())
+    Fn->addFnAttr("patchable-function", "prologue-short-redirect");
 
   // Add no-jump-tables value.
   if (CGM.getCodeGenOpts().NoUseJumpTables)