Compiler crash on MSVC when building with JIT and PGO

[mdboom]

Bug report

Bug description:

Building with the JIT and PGO simultaneously causes MSVC 1941 to crash:

Merging D:\a\cpython\cpython\PCbuild\amd64\python314!1.pgc
  D:\a\cpython\cpython\PCbuild\amd64\python314!1.pgc: Used 31.3% (19698312 / 62844928) of total space reserved.  0.0% of the counts were dropped due to overflow.
    Reading PGD file 1: D:\a\cpython\cpython\PCbuild\amd64\python314.pgd
     Creating library D:\a\cpython\cpython\PCbuild\amd64\python314.lib and object D:\a\cpython\cpython\PCbuild\amd64\python314.exp
  Generating code
  
  0 of 0 ( 0.0%) original invalid call sites were matched.
  0 new call sites were added.
  291 of 16259 (  1.79%) profiled functions will be compiled for speed, and the rest of the functions will be compiled for size
D:\a\cpython\cpython\Python\ceval.c(766): fatal error C1001: Internal compiler error. [D:\a\cpython\cpython\PCbuild\pythoncore.vcxproj]
  (compiler file 'D:\a\_work\1\s\src\vctools\Compiler\Utc\src\p2\main.c', line 247)
   To work around this problem, try simplifying or changing the program near the locations listed above.
  If possible please provide a repro here: https://developercommunity.visualstudio.com/ 
  Please choose the Technical Support command on the Visual C++ 
   Help menu, or open the Technical Support help file for more information
    link!InvokeCompilerPass()+0xd94ce
    link!InvokeCompilerPass()+0xd94ce
    link!InvokeCompilerPass()+0xd922d
    link!InvokeCompilerPass()+0xd610c
    link!InvokeCompilerPass()+0xd6011
    link!CloseTypeServerPDB()+0x12904
    link!InvokeCompilerPassW()+0x65cd9
    link!InvokeCompilerPassW()+0x65c9c
    link!time32()+0x83
    link!BaseThreadInitThunk()+0x19
    link!RtlGetFullPathName_UEx()+0xad
    link!RtlGetFullPathName_UEx()+0x7b

CPython versions tested on:

CPython main branch

Operating systems tested on:

Windows

Linked PRs

• gh-125217: Turn off optimization around_PyEval_EvalFrameDefault to avoid MSVC crash #125218
• gh-125217: Turn off optimization around_PyEval_EvalFrameDefault to avoid MSVC crash #125477

[mdboom]

Perhaps this is the same root cause as the crash with the free-threading build as well (just extra complexity in _PyEval_EvalFrameDefault #123153.

[mdboom]

This problem is proving tricky to solve. In the past when we've seen this crash, it has been enough to disable speed optimizations on the single _PyEval_EvalFrameDefault function using #pragma optimize("t", off), but that's not working in this case. I have also tried turning off all optimizations on that function to no avail.

Just to confirm why PGO is so important, I measured its effect on the last known-working commit (04c837d) and PGO improves speed by about 14% on the pyperformance suite. So "just not using PGO on Windows" isn't a viable option.

git bisect tells me that the first commit to cause the compiler to crash with the JIT/PGO combination is da071fa #124392, which at least seems plausible. Maybe there is something we can do with those changes to workaround the compiler crash (Cc: @markshannon) as a stopgap solution.

In the meantime, now that I have a set of changes to point to, I can file a bug against MSVC.

[mdboom]

Here is the bug I filed against MSVC: https://developercommunity.visualstudio.com/t/C1001-when-compiling-very-large-function/10767031

[neonene]

Leaving a few possible alternatives just in case:

_PyEval_EvalFrameDefault(...)
{
 ...
-    uint8_t opcode;    /* Current opcode */
+    uint16_t opcode;   /* Current opcode */
 ...
 #if USE_COMPUTED_GOTOS
         _unknown_opcode:
 #else
-        EXTRA_CASES  // From pycore_opcode_metadata.h, ...
+        default:
 #endif

Adding an extra opcode case more than 299, which loses the advantage of EXTRA_CASES:

_PyEval_EvalFrameDefault(...)
{
 ...

 #if USE_COMPUTED_GOTOS
         _unknown_opcode:
 #else
         EXTRA_CASES  // From pycore_opcode_metadata.h, ...
+        case 500:
 #endif

I guess MSVC might want to merge the jump tables.