Avoid unnecessary branches when fixing non-null/empty arrays (#2668)

When we know an array is not null or empty at the point where fixed is
used, we can reduce the number of branches (and IL size of the method
body) by using the address of the first element.

src/Common/src/Interop/Unix/System.Native/Interop.GetCwd.cs

@@ -34,7 +34,7 @@ internal static unsafe string GetCwd()
                 {
                     checked { bufferSize *= 2; }
                     var buf = new byte[Math.Min(bufferSize, maxPath)];
-                    fixed (byte* ptr = buf)
+                    fixed (byte* ptr = &buf[0])
                     {
                         result = GetCwdHelper(ptr, buf.Length);
                         if (result != null)

src/System.Private.CoreLib/src/Internal/Runtime/CompilerHelpers/InteropHelpers.cs

@@ -28,7 +28,7 @@ internal static unsafe byte[] StringToAnsi(String str)
                 int stringLength = str.Length;
                 int bufferLength = encoding.GetByteCount(pStr, stringLength);
                 var buffer = new byte[bufferLength + 1];
-                fixed (byte* pBuffer = buffer)
+                fixed (byte* pBuffer = &buffer[0])
                 {
                     encoding.GetBytes(pStr, stringLength, pBuffer, bufferLength);
                     return buffer;

src/System.Private.CoreLib/src/System/Array.cs

@@ -2532,18 +2532,22 @@ public unsafe Object GetValue(params int[] indices)
             if (indices == null)
                 throw new ArgumentNullException(nameof(indices));
 
-            if (IsSzArray && indices.Length == 1)
+            int length = indices.Length;
+
+            if (IsSzArray && length == 1)
                 return GetValue(indices[0]);
 
-            fixed (int* pIndices = indices)
-                return GetValue(pIndices, indices.Length);
+            if (Rank != length)
+                throw new ArgumentException(SR.Arg_RankIndices);
+
+            Debug.Assert(length > 0);
+            fixed (int* pIndices = &indices[0])
+                return GetValue(pIndices, length);
         }
 
         private unsafe Object GetValue(int* pIndices, int rank)
         {
-            if (this.Rank != rank)
-                throw new ArgumentException(SR.Arg_RankIndices);
-
+            Debug.Assert(Rank == rank);
             Debug.Assert(!IsSzArray);
 
             fixed (IntPtr* pThisArray = &m_pEEType)
@@ -2662,24 +2666,28 @@ public unsafe void SetValue(Object value, params int[] indices)
             if (indices == null)
                 throw new ArgumentNullException(nameof(indices));
 
-            if (IsSzArray && indices.Length == 1)
+            int length = indices.Length;
+
+            if (IsSzArray && length == 1)
             {
                 SetValue(value, indices[0]);
                 return;
             }
 
-            fixed (int* pIndices = indices)
+            if (Rank != length)
+                throw new ArgumentException(SR.Arg_RankIndices);
+
+            Debug.Assert(length > 0);
+            fixed (int* pIndices = &indices[0])
             {
-                SetValue(value, pIndices, indices.Length);
+                SetValue(value, pIndices, length);
                 return;
             }
         }
 
         private unsafe void SetValue(Object value, int* pIndices, int rank)
         {
-            if (this.Rank != rank)
-                throw new ArgumentException(SR.Arg_RankIndices);
-
+            Debug.Assert(Rank == rank);
             Debug.Assert(!IsSzArray);
 
             fixed (IntPtr* pThisArray = &m_pEEType)

src/System.Private.CoreLib/src/System/Convert.cs

@@ -2392,7 +2392,7 @@ internal static unsafe String ToBase64String(byte[] inArray, int offset, int len
             string returnString = new string('\0', stringLength);
             fixed (char* outChars = returnString)
             {
-                fixed (byte* inData = inArray)
+                fixed (byte* inData = &inArray[0])
                 {
                     int j = ConvertToBase64Array(outChars, inData, offset, length, insertLineBreaks);
                     System.Diagnostics.Debug.Assert(returnString.Length == j, "returnString.Length == j");
@@ -2459,7 +2459,7 @@ internal static unsafe int ToBase64CharArray(byte[] inArray, int offsetIn, int l
 
             fixed (char* outChars = &outArray[offsetOut])
             {
-                fixed (byte* inData = inArray)
+                fixed (byte* inData = &inArray[0])
                 {
                     retVal = ConvertToBase64Array(outChars, inData, offsetIn, length, insertLineBreaks);
                 }
@@ -2478,7 +2478,7 @@ private static unsafe int ConvertToBase64Array(char* outChars, byte* inData, int
             int i;
 
             // get a pointer to the base64Table to avoid unnecessary range checking
-            fixed (char* base64 = base64Table)
+            fixed (char* base64 = &base64Table[0])
             {
                 for (i = offset; i < calcLength; i += 3)
                 {

src/System.Private.CoreLib/src/System/Environment.Win32.cs

@@ -43,7 +43,7 @@ public static unsafe String GetEnvironmentVariable(String variable)
                 currentSize = requiredSize;
                 // need to retry since the environment variable might be changed 
                 newblob = new char[currentSize];
-                fixed (char* pText = variable, pBlob = newblob)
+                fixed (char* pText = variable, pBlob = &newblob[0])
                 {
                     requiredSize = Interop.mincore.GetEnvironmentVariable(pText, pBlob, currentSize);
                 }

src/System.Private.CoreLib/src/System/Exception.cs

@@ -500,7 +500,7 @@ internal unsafe byte[] SerializeForDump()
                 int cbBuffer = sizeof(SERIALIZED_EXCEPTION_HEADER) + (nStackTraceElements * IntPtr.Size);
 
                 byte[] buffer = new byte[cbBuffer];
-                fixed (byte* pBuffer = buffer)
+                fixed (byte* pBuffer = &buffer[0])
                 {
                     SERIALIZED_EXCEPTION_HEADER* pHeader = (SERIALIZED_EXCEPTION_HEADER*)pBuffer;
                     pHeader->HResult = _HResult;

src/System.Private.CoreLib/src/System/Globalization/CharUnicodeInfo.cs

@@ -178,7 +178,7 @@ internal static unsafe double InternalGetNumericValue(int ch)
             fixed (ushort* pUshortPtr = &(s_pNumericLevel1Index[index]))
             {
                 byte* pBytePtr = (byte*)pUshortPtr;
-                fixed (byte* pByteNum = s_pNumericValues)
+                fixed (byte* pByteNum = &s_pNumericValues[0])
                 {
                     double* pDouble = (double*)pByteNum;
                     return pDouble[pBytePtr[(ch & 0x000f)]];

src/System.Private.CoreLib/src/System/Globalization/CompareInfo.Unix.cs

@@ -347,7 +347,7 @@ internal unsafe int GetHashCodeOfStringCore(string source, CompareOptions option
 
             byte[] sortKey = new byte[sortKeyLength];
 
-            fixed(byte* pSortKey = sortKey)
+            fixed (byte* pSortKey = &sortKey[0])
             {
                 Interop.GlobalizationInterop.GetSortKey(_sortHandle, source, source.Length, pSortKey, sortKeyLength, options);
                 return InternalHashSortKey(pSortKey, sortKeyLength, false, additionalEntropy);

src/System.Private.CoreLib/src/System/Globalization/FormatProvider.DateTimeFormat.cs

@@ -188,7 +188,7 @@ internal static unsafe void FormatDigits(StringBuilder outputBuffer, int value,
                 int digits;
 
                 char[] charBuff = new char[16];
-                fixed (char* buffer = charBuff)
+                fixed (char* buffer = &charBuff[0])
                 {
                     char* p = buffer + 16;
                     int n = value;

src/System.Private.CoreLib/src/System/Globalization/IdnMapping.Unix.cs

@@ -35,7 +35,7 @@ private unsafe string GetAsciiCore(char* unicode, int count)
             }
 
             char[] outputHeap = new char[actualLength];
-            fixed (char* pOutputHeap = outputHeap)
+            fixed (char* pOutputHeap = &outputHeap[0])
             {
                 actualLength = Interop.GlobalizationInterop.ToAscii(flags, unicode, count, pOutputHeap, actualLength);
                 if (actualLength == 0 || actualLength > outputHeap.Length)
@@ -60,7 +60,7 @@ private unsafe string GetUnicodeCore(char* ascii, int count)
             else
             {
                 char[] output = new char[count];
-                fixed (char* pOutput = output)
+                fixed (char* pOutput = &output[0])
                 {
                     return GetUnicodeCore(ascii, count, flags, pOutput, count, reattempt: true);
                 }

src/System.Private.CoreLib/src/System/Globalization/IdnMapping.Windows.cs

@@ -30,7 +30,7 @@ private unsafe string GetAsciiCore(char* unicode, int count)
             else
             {
                 char[] output = new char[length];
-                fixed (char* pOutput = output)
+                fixed (char* pOutput = &output[0])
                 {
                     return GetAsciiCore(unicode, count, flags, pOutput, length);
                 }
@@ -69,7 +69,7 @@ private unsafe string GetUnicodeCore(char* ascii, int count)
             else
             {
                 char[] output = new char[length];
-                fixed (char* pOutput = output)
+                fixed (char* pOutput = &output[0])
                 {
                     return GetUnicodeCore(ascii, count, flags, pOutput, length);
                 }

src/System.Private.CoreLib/src/System/IO/FileStream.Win32.cs

@@ -1284,7 +1284,7 @@ private unsafe int ReadFileNative(SafeFileHandle handle, byte[] bytes, int offse
             int r = 0;
             int numBytesRead = 0;
 
-            fixed (byte* p = bytes)
+            fixed (byte* p = &bytes[0])
             {
                 if (_useAsyncIO)
                     r = Interop.mincore.ReadFile(handle, p + offset, count, IntPtr.Zero, overlapped);
@@ -1329,7 +1329,7 @@ private unsafe int WriteFileNative(SafeFileHandle handle, byte[] bytes, int offs
             int numBytesWritten = 0;
             int r = 0;
 
-            fixed (byte* p = bytes)
+            fixed (byte* p = &bytes[0])
             {
                 if (_useAsyncIO)
                     r = Interop.mincore.WriteFile(handle, p + offset, count, IntPtr.Zero, overlapped);

src/System.Private.CoreLib/src/System/Runtime/RuntimeImports.cs

@@ -569,7 +569,7 @@ internal static byte[] ConvertPublicKeyToPublicKeyToken(byte[] publicKey)
             {
                 fixed (byte* pPublicKey = publicKey)
                 {
-                    fixed (byte* pPublicKeyToken = publicKeyToken)
+                    fixed (byte* pPublicKeyToken = &publicKeyToken[0])
                     {
                         RhConvertPublicKeyToPublicKeyToken(pPublicKey, publicKey.Length, pPublicKeyToken, publicKeyToken.Length);
                     }

src/System.Private.CoreLib/src/System/RuntimeExceptionHelpers.cs

@@ -332,7 +332,7 @@ public unsafe byte[] Serialize()
                 checked
                 {
                     byte[] serializedData = new byte[sizeof(ExceptionMetadataStruct) + SerializedExceptionData.Length];
-                    fixed (byte* pSerializedData = serializedData)
+                    fixed (byte* pSerializedData = &serializedData[0])
                     {
                         ExceptionMetadataStruct* pMetadata = (ExceptionMetadataStruct*)pSerializedData;
                         pMetadata->ExceptionId = ExceptionMetadata.ExceptionId;
@@ -538,7 +538,7 @@ private static unsafe void GenerateErrorReportForDump(LowLevelList<byte[]> seria
                 }
 
                 byte[] finalBuffer = new byte[cbFinalBuffer];
-                fixed (byte* pBuffer = finalBuffer)
+                fixed (byte* pBuffer = &finalBuffer[0])
                 {
                     byte* pCursor = pBuffer;
                     int cbRemaining = cbFinalBuffer;
@@ -593,17 +593,19 @@ public static bool SafeToPerformRichExceptionSupport
 
         private static unsafe void UpdateErrorReportBuffer(byte[] finalBuffer)
         {
+            Debug.Assert(finalBuffer?.Length > 0);
+
             using (LockHolder.Hold(s_ExceptionInfoBufferLock))
             {
-                fixed (byte* pBuffer = finalBuffer)
+                fixed (byte* pBuffer = &finalBuffer[0])
                 {
                     byte* pPrevBuffer = (byte*)RuntimeImports.RhSetErrorInfoBuffer(pBuffer);
                     Debug.Assert(s_ExceptionInfoBufferPinningHandle.IsAllocated == (pPrevBuffer != null));
                     if (pPrevBuffer != null)
                     {
                         byte[] currentExceptionInfoBuffer = (byte[])s_ExceptionInfoBufferPinningHandle.Target;
-                        Debug.Assert(currentExceptionInfoBuffer != null);
-                        fixed (byte* pPrev = currentExceptionInfoBuffer)
+                        Debug.Assert(currentExceptionInfoBuffer?.Length > 0);
+                        fixed (byte* pPrev = &currentExceptionInfoBuffer[0])
                             Debug.Assert(pPrev == pPrevBuffer);
                     }
                     if (!s_ExceptionInfoBufferPinningHandle.IsAllocated)

src/System.Private.CoreLib/src/System/String.cs

@@ -350,7 +350,7 @@ unsafe public char[] ToCharArray()
             {
                 char[] chars = new char[length];
                 fixed (char* src = &_firstChar)
-                    fixed (char* dest = chars)
+                    fixed (char* dest = &chars[0])
                 {
                     wstrcpy(dest, src, length);
                 }
@@ -373,7 +373,7 @@ unsafe public char[] ToCharArray(int startIndex, int length)
             {
                 char[] chars = new char[length];
                 fixed (char* src = &_firstChar)
-                    fixed (char* dest = chars)
+                    fixed (char* dest = &chars[0])
                 {
                     wstrcpy(dest, src + startIndex, length);
                 }

src/System.Private.CoreLib/src/System/Text/DecoderNLS.cs

@@ -75,7 +75,7 @@ public override unsafe int GetCharCount(byte[] bytes, int index, int count, bool
                 bytes = new byte[1];
 
             // Just call pointer version
-            fixed (byte* pBytes = bytes)
+            fixed (byte* pBytes = &bytes[0])
                 return GetCharCount(pBytes + index, count, flush);
         }
 
@@ -136,8 +136,8 @@ public override unsafe int GetChars(byte[] bytes, int byteIndex, int byteCount,
                 chars = new char[1];
 
             // Just call pointer version
-            fixed (byte* pBytes = bytes)
-                fixed (char* pChars = chars)
+            fixed (byte* pBytes = &bytes[0])
+                fixed (char* pChars = &chars[0])
                     // Remember that charCount is # to decode, not size of array
                     return GetChars(pBytes + byteIndex, byteCount,
                                     pChars + charIndex, charCount, flush);
@@ -200,9 +200,9 @@ public override unsafe void Convert(byte[] bytes, int byteIndex, int byteCount,
                 chars = new char[1];
 
             // Just call the pointer version (public overrides can't do this)
-            fixed (byte* pBytes = bytes)
+            fixed (byte* pBytes = &bytes[0])
             {
-                fixed (char* pChars = chars)
+                fixed (char* pChars = &chars[0])
                 {
                     Convert(pBytes + byteIndex, byteCount, pChars + charIndex, charCount, flush,
                         out bytesUsed, out charsUsed, out completed);

src/System.Private.CoreLib/src/System/Text/EncoderNLS.cs

@@ -74,7 +74,7 @@ public override unsafe int GetByteCount(char[] chars, int index, int count, bool
 
             // Just call the pointer version
             int result = -1;
-            fixed (char* pChars = chars)
+            fixed (char* pChars = &chars[0])
             {
                 result = GetByteCount(pChars + index, count, flush);
             }
@@ -127,8 +127,8 @@ public override unsafe int GetBytes(char[] chars, int charIndex, int charCount,
                 bytes = new byte[1];
 
             // Just call pointer version
-            fixed (char* pChars = chars)
-                fixed (byte* pBytes = bytes)
+            fixed (char* pChars = &chars[0])
+                fixed (byte* pBytes = &bytes[0])
 
                     // Remember that charCount is # to decode, not size of array.
                     return GetBytes(pChars + charIndex, charCount,
@@ -188,9 +188,9 @@ public override unsafe void Convert(char[] chars, int charIndex, int charCount,
                 bytes = new byte[1];
 
             // Just call the pointer version (can't do this for non-msft encoders)
-            fixed (char* pChars = chars)
+            fixed (char* pChars = &chars[0])
             {
-                fixed (byte* pBytes = bytes)
+                fixed (byte* pBytes = &bytes[0])
                 {
                     Convert(pChars + charIndex, charCount, pBytes + byteIndex, byteCount, flush,
                         out charsUsed, out bytesUsed, out completed);

src/System.Private.CoreLib/src/System/Text/EncodingForwarder.cs

@@ -129,7 +129,7 @@ public static unsafe int GetBytes(Encoding encoding, string s, int charIndex, in
             if (bytes.Length == 0)
                 bytes = new byte[1];
 
-            fixed (char* pChars = s) fixed (byte* pBytes = bytes)
+            fixed (char* pChars = s) fixed (byte* pBytes = &bytes[0])
             {
                 return encoding.GetBytes(pChars + charIndex, charCount, pBytes + byteIndex, byteCount, encoder: null);
             }
@@ -169,7 +169,7 @@ public static unsafe int GetBytes(Encoding encoding, char[] chars, int charIndex
                 bytes = new byte[1];
 
             // Just call the (internal) pointer version
-            fixed (char* pChars = chars) fixed (byte* pBytes = bytes)
+            fixed (char* pChars = chars) fixed (byte* pBytes = &bytes[0])
             {
                 return encoding.GetBytes(pChars + charIndex, charCount, pBytes + byteIndex, byteCount, encoder: null);
             }
@@ -265,7 +265,7 @@ public static unsafe int GetChars(Encoding encoding, byte[] bytes, int byteIndex
             if (chars.Length == 0)
                 chars = new char[1];
 
-            fixed (byte* pBytes = bytes) fixed (char* pChars = chars)
+            fixed (byte* pBytes = bytes) fixed (char* pChars = &chars[0])
             {
                 return encoding.GetChars(pBytes + byteIndex, byteCount, pChars + charIndex, charCount, decoder: null);
             }

src/System.Private.Interop/src/Interop/Interop.Localization.Windows.cs

@@ -14,6 +14,7 @@
 // always manually marshal the arguments
 
 using System;
+using System.Diagnostics;
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
 
@@ -90,10 +91,11 @@ internal static extern unsafe int FormatMessage(
         /// <returns>Return false IFF when buffer space isnt enough</returns>
         private static unsafe bool TryGetMessage(int errorCode, int bufferSize, out string errorMsg)
         {
+            Debug.Assert(bufferSize > 0);
             errorMsg = null;
             char[] buffer = new char[bufferSize];
             int result;
-            fixed (char* pinned_lpBuffer = buffer)
+            fixed (char* pinned_lpBuffer = &buffer[0])
             {
                 result = ExternalInterop.FormatMessage(
                     FORMAT_MESSAGE_IGNORE_INSERTS | FORMAT_MESSAGE_FROM_SYSTEM |