Less optimistic checking for const vector to pattern checks

src/coreclr/jit/simd.h

@@ -1976,49 +1976,59 @@ SveMaskPattern EvaluateSimdMaskToPattern(simdmask_t arg0)
 
     uint64_t mask;
     memcpy(&mask, &arg0.u8[0], sizeof(uint64_t));
-    uint32_t finalOne = count;
+    uint32_t firstZero = count;
 
-    // A mask pattern starts with zero of more 1s and then the rest of the mask is filled with 0s.
+    constexpr uint64_t laneMask = (sizeof(TBase) == 8) ? ~0ull : ((1ull << sizeof(TBase)) - 1ull);
+
+    // A mask is a vector of unsigned integers, where 1 indicates the lane is set, 0 is not set,
+    // and all other values are undefined.
+    // For a valid mask pattern:
+    // * Each element of size TBase contains 0 or 1 in the lowest bit, and no other bits set.
+    // * The sequence starts with zero or more 1s and then the rest of the mask is filled with 0s.
 
     // Find an unbroken sequence of 1s.
     for (uint32_t i = 0; i < count; i++)
     {
-        // For Arm64 we have count total bits to read, but
-        // they are sizeof(TBase) bits apart. We set
-        // the result element to AllBitsSet or Zero depending
-        // on the corresponding mask bit
+        const uint64_t lane = (mask >> static_cast<uint32_t>(i * sizeof(TBase)));
+        TBase          elem = (TBase)(lane & laneMask);
 
-        bool isSet = ((mask >> (i * sizeof(TBase))) & 1) != 0;
-        if (!isSet)
+        if (elem == 0)
         {
-            finalOne = i;
+            // Found the first zero
+            firstZero = i;
             break;
         }
+        else if (elem != 1)
+        {
+            // Other bits are set. Invalid sequence
+            return SveMaskPatternNone;
+        }
+        // else just bit 1 in elem was set
     }
 
     // Find an unbroken sequence of 0s.
-    for (uint32_t i = finalOne; i < count; i++)
+    for (uint32_t i = firstZero; i < count; i++)
     {
-        // For Arm64 we have count total bits to read, but
-        // they are sizeof(TBase) bits apart. We set
-        // the result element to AllBitsSet or Zero depending
-        // on the corresponding mask bit
+        const uint64_t lane = (mask >> static_cast<uint32_t>(i * sizeof(TBase)));
+        TBase          elem = (TBase)(lane & laneMask);
 
-        bool isSet = ((mask >> (i * sizeof(TBase))) & 1) != 0;
-        if (isSet)
+        if (elem != 0)
         {
-            // Invalid sequence
+            // Either a 1 or other bits are set. Invalid sequence
             return SveMaskPatternNone;
         }
     }
 
-    if (finalOne == count)
+    assert(firstZero <= count);
+
+    if (firstZero == count)
     {
+        // No zeros in the pattern
         return SveMaskPatternAll;
     }
-    else if (finalOne >= SveMaskPatternVectorCount1 && finalOne <= SveMaskPatternVectorCount8)
+    else if (firstZero >= SveMaskPatternVectorCount1 && firstZero <= SveMaskPatternVectorCount8)
     {
-        return (SveMaskPattern)finalOne;
+        return (SveMaskPattern)firstZero;
     }
     else
     {

src/tests/JIT/Regression/JitBlue/Runtime_124511/Runtime_124511.cs

@@ -0,0 +1,38 @@
+// Generated by Fuzzlyn v3.3 on 2026-02-15 17:21:42
+// Run on Arm64 Linux
+// Seed: 103488890012433678-vectort,vector64,vector128,armadvsimd,armadvsimdarm64,armaes,armarmbase,armarmbasearm64,armcrc32,armcrc32arm64,armdp,armrdm,armrdmarm64,armsha1,armsha256,armsve,armsve2
+// Reduced from 29.2 KiB to 0.7 KiB in 00:00:16
+// Debug: Outputs -3
+// Release: Outputs -2
+
+namespace Runtime_124511;
+
+using System.Numerics;
+using System.Runtime.CompilerServices;
+using System.Runtime.Intrinsics;
+using System.Runtime.Intrinsics.Arm;
+using Xunit;
+
+public unsafe class Runtime_124511
+{
+    [ConditionalFact(typeof(Sve), nameof(Sve.IsSupported))]
+    public static void TestEntryPoint()
+    {
+        Assert.True(MainTest() == -3);
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    public static sbyte MainTest()
+    {
+        var vr8 = Vector.Create<uint>(1);
+        var vr9 = Vector128.CreateScalar(4701966028735610216UL).AsVector();
+        var vr10 = Sve.ShiftLeftLogical(vr8, vr9);
+        var vr11 = Vector128.CreateScalar(2U).AsVector();
+        var vr12 = Sve.Subtract(vr10, vr11);
+        var vr13 = Vector.Create<uint>(1);
+        var vr14 = Vector128.CreateScalar(0U).AsVector();
+        var vr15 = Sve.CreateBreakBeforePropagateMask(vr12, vr13, vr14);
+        sbyte vr16 = (sbyte)Sve.SaturatingDecrementByActiveElementCount(0, vr15);
+        return vr16;
+    }
+}

src/tests/JIT/Regression/Regression_ro_1.csproj

@@ -78,6 +78,7 @@
     <Compile Include="JitBlue\Runtime_121711\Runtime_121711.cs" />
     <Compile Include="JitBlue\Runtime_122138\Runtime_122138.cs" />
     <Compile Include="JitBlue\Runtime_123399\Runtime_123399.cs" />
+    <Compile Include="JitBlue\Runtime_124511\Runtime_124511.cs" />
     <Compile Include="JitBlue\Runtime_31615\Runtime_31615.cs" />
     <Compile Include="JitBlue\Runtime_33884\Runtime_33884.cs" />
     <Compile Include="JitBlue\Runtime_38920\Runtime_38920.cs" />