[llvm-dev] Using llc -disablg-cgp to fix instruction selection (store) error

[Alex Susu via llvm-dev]

   Hello.
     I want to report a bug I encountered and the quick fix I found.
     Basically I was getting the following rather strange instruction selection error for 
a program using vector LLVM IR instructions when giving llc -O1 (but not for -O0):
         ISEL: Starting pattern match on root node: t8965: ch = store<ST256[FixedStack0]> 
t0, t1068, FrameIndex:i64<0>, undef:i64
           Initial Opcode index to 157
           Skipped scope entry (due to false predicate) at index 162, continuing at 236
           Match failed at index 241
           Continuing at 263
         LLVM ERROR: Cannot select: t8965: ch = store<ST256[FixedStack0]> t0, t1068, 
FrameIndex:i64<0>, undef:i64
           t1068: v128i16 = add t9412, t9540
             t9412: v128i16 = insert_vector_elt t9411, t5182, Constant:i64<127>
               t9411: v128i16 = insert_vector_elt t9410, t5184, Constant:i64<126>
                 t9410: v128i16 = insert_vector_elt t9409, t5186, Constant:i64<125>
                   t9409: v128i16 = insert_vector_elt t9408, t5188, Constant:i64<124>
                     t9408: v128i16 = insert_vector_elt t9407, t5190, Constant:i64<123>
                       t9407: v128i16 = insert_vector_elt t9406, t5192, Constant:i64<122>
                         t9406: v128i16 = insert_vector_elt t9405, t5194, Constant:i64<121>
                           t9405: v128i16 = insert_vector_elt t9404, t5196, Constant:i64<120>

     Note: I saw also that my vector loads and stores (gathers and scatters, to be more 
exact) are treated differently: LD256 is broken into several LD2 instructions; ST256 has 
FrameIndex / FixedStack0 attributes now (for llc -O0 I think it does not).

     I had the inspiration to examine the standard output of llc around a message "CGP: 
Found ...", which was something new from llc -O0. This message is generated by method 
CodeGenPrepare::optimizeMemoryInst(), which "is used to optimize both load/store and 
inline asms with memory operands."

     So I gave llc -O1 -disable-cgp to Disable Codegen Prepare and I got NO more error at 
llc -O1.

     Any comment on this solved issue is appreciated.

   Best regards,
     Alex