Force enable SSE2 on RyuJIT/x86 on desktop

SSE2 is required for RyuJIT/x86; there is no x87 code path.
The .NET Core VM sets the JIT flag enabling this, and we assert
it does. On desktop, however, especially under NGEN, it does not.
We ignore that, and generate SSE2 code anyway.

diff --git a/src/jit/compiler.cpp b/src/jit/compiler.cpp
index 999ca9e..1d7e280 100644 (file)
--- a/src/jit/compiler.cpp
+++ b/src/jit/compiler.cpp
@@ -2270,7 +2270,7 @@ void Compiler::compSetProcessor()
 #if defined(_TARGET_ARM_)
     info.genCPU = CPU_ARM;
 #elif defined(_TARGET_AMD64_)
-    info.genCPU         = CPU_X64;
+    info.genCPU       = CPU_X64;
 #elif defined(_TARGET_X86_)
     if (jitFlags.IsSet(JitFlags::JIT_FLAG_TARGET_P4))
         info.genCPU = CPU_X86_PENTIUM_4;
@@ -2307,20 +2307,32 @@ void Compiler::compSetProcessor()
     opts.compUseCMOV    = true;
     opts.compCanUseSSE2 = true;
 #elif defined(_TARGET_X86_)
-    opts.compUseFCOMI   = jitFlags.IsSet(JitFlags::JIT_FLAG_USE_FCOMI);
-    opts.compUseCMOV    = jitFlags.IsSet(JitFlags::JIT_FLAG_USE_CMOV);
+    opts.compUseFCOMI = jitFlags.IsSet(JitFlags::JIT_FLAG_USE_FCOMI);
+    opts.compUseCMOV  = jitFlags.IsSet(JitFlags::JIT_FLAG_USE_CMOV);
     opts.compCanUseSSE2 = jitFlags.IsSet(JitFlags::JIT_FLAG_USE_SSE2);
 
+#if !defined(LEGACY_BACKEND) && !defined(FEATURE_CORECLR)
+    // RyuJIT/x86 requires SSE2 to be available: there is no support for generating floating-point
+    // code with x87 instructions. On .NET Core, the VM always tells us that SSE2 is available.
+    // However, on desktop, under ngen, (and presumably in the unlikely case you're actually
+    // running on a machine without SSE2), the VM does not set the SSE2 flag. We ignore this and
+    // go ahead and generate SSE2 code anyway.
+    if (!opts.compCanUseSSE2)
+    {
+        JITDUMP("VM didn't set CORJIT_FLG_USE_SSE2! Ignoring, and generating SSE2 code anyway.\n");
+        opts.compCanUseSSE2 = true;
+    }
+#endif // !defined(LEGACY_BACKEND) && !defined(FEATURE_CORECLR)
+
 #ifdef DEBUG
     if (opts.compUseFCOMI)
         opts.compUseFCOMI = !compStressCompile(STRESS_USE_FCOMI, 50);
     if (opts.compUseCMOV)
         opts.compUseCMOV = !compStressCompile(STRESS_USE_CMOV, 50);
 
-// X86 RyuJIT requires SSE2 to be available and hence
-// don't turn off compCanUseSSE2 under stress.
 #ifdef LEGACY_BACKEND
-    // Should we override the SSE2 setting
+
+    // Should we override the SSE2 setting?
     enum
     {
         SSE2_FORCE_DISABLE = 0,
@@ -2334,9 +2346,14 @@ void Compiler::compSetProcessor()
         opts.compCanUseSSE2 = true;
     else if (opts.compCanUseSSE2)
         opts.compCanUseSSE2 = !compStressCompile(STRESS_GENERIC_VARN, 50);
-#else  // !LEGACY_JITBACKEND
+
+#else // !LEGACY_BACKEND
+
+    // RyuJIT/x86 requires SSE2 to be available and hence
+    // don't turn off compCanUseSSE2 under stress.
     assert(opts.compCanUseSSE2);
-#endif // !LEGACY_JITBACKEND
+
+#endif // !LEGACY_BACKEND
 
 #endif // DEBUG