[PATCH] D79537: Add NoMerge MIFlag to avoid MIR branch folding

Wed May 13 14:10:10 PDT 2020

zequanwu added a comment.

In D79537#2035041 <https://reviews.llvm.org/D79537#2035041>, @rnk wrote:

> After this lands, can you follow this up by migrating the sanitizer passes to using the attribute instead of using empty inline asm blobs? Send the patch to @vitalybuka. This is the code I'm thinking of:
>  https://github.com/llvm/llvm-project/blob/028bfdd8913616f7a3e57e8ef5c2a9990e528ff0/llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp#L1626
>  https://github.com/llvm/llvm-project/blob/d6695e18763a05b30cb336c18157175277da8f4b/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp#L864
>  ... and any similar uses.
>
> Run `check-asan`, `check-msan`, etc to run the sanitizer integration tests, and confirm that they pass. You can run `check-all` to run everything and be sure as well.

Sure, I will do after this lands.

================
Comment at: llvm/include/llvm/CodeGen/MachineInstr.h:109
                                         // floatint-point exceptions.
+    NoMerge      = 1 << 15,             // Instruction should not be merged
   };
----------------
rnk wrote:
> arsenm wrote:
> > It's not obvious to me what this means. These are also supposed to be droppable optimization flags, but it seems like you're using this for something semantically required?
> I don't think `BundledPred` or `FrameSetup` are droppable, IMO they are semantically significant.
> 
> The only alternative I see to this would be to encode this information in the MCInstrDesc, which would require mirroring the entire family of X86 CALL instructions, of which there are many.
This `NoMerge` flag is used to disable branch-fold optimization when present. What do you mean something semantically required?

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D79537/new/

https://reviews.llvm.org/D79537