Enable legacy JIT fallback for CoreCLR on Windows x86

This is to support the eventual case where RyuJIT/x86 becomes the default
x86 JIT, but we support fallback to the older, legacy JIT32/x86 (which
will be renamed compatjit.dll). This uses the same mechanism that was
built for .NET 4.6 when RyuJIT/x64 was shipped, and allowed fallback
to JIT64 as compatjit.dll.

However, we use a different configuration name to avoid conflicting
with the .NET 4.6 COMPlus_useLegacy name (and unintentionally causing
users to fall back to JIT64 with their .NET 4.6 apps).

The COMPlus variable is COMPlus_UseWindowsX86CoreLegacyJit=1. For the
dotnet.exe host, you can set `"System.JIT.UseWindowsX86CoreLegacyJit": true`
in the "configProperties" section of the app.runtimeconfig.json file.

There is a new COMPlus_RequireLegacyJit=1 option to aid testing JIT
fallback.

1 files changed, 7 insertions, 1 deletions

diff --git a/src/vm/codeman.h b/src/vm/codeman.h
index aaf1be8bbf..f4e04b8c67 100644
--- a/src/vm/codeman.h
+++ b/src/vm/codeman.h
@@ -1193,10 +1193,16 @@ public:
 public:
     ICorJitCompiler *   m_jit;
     HINSTANCE           m_JITCompiler;
-#ifdef _TARGET_AMD64_
+#if defined(_TARGET_X86_) || defined(_TARGET_AMD64_)
     HINSTANCE           m_JITCompilerOther; // Stores the handle of the legacy JIT, if one is loaded.
 #endif
 
+    // TRUE if the legacy/compat JIT was loaded successfully and will be used.
+    // This is available in all builds so if COMPlus_RequireLegacyJit=1 is set in a test,
+    // the test will fail in any build where the legacy JIT is not loaded, even if legacy
+    // fallback is not available in that build. This prevents unexpected silent successes.
+    BOOL                m_fLegacyJitUsed;
+
 #ifdef ALLOW_SXS_JIT
     //put these at the end so that we don't mess up the offsets in the DAC.
     ICorJitCompiler *   m_alternateJit;