From ccee78d2d423649000f5f0d0c1ece1ccd4d95f51 Mon Sep 17 00:00:00 2001
From: nathan-moore <nathanmoore424@gmail.com>
Date: Mon, 21 Sep 2020 02:58:52 -0400
Subject: [PATCH] emit test for bounds checks against a 0 index (#42295)

* emit test for bounds checks against 0

* formatting

* remove dead code

* formatting
---
 src/coreclr/src/jit/codegenxarch.cpp | 26 +++++++++++++++++++-------
 1 file changed, 19 insertions(+), 7 deletions(-)

diff --git a/src/coreclr/src/jit/codegenxarch.cpp b/src/coreclr/src/jit/codegenxarch.cpp
index db6a880..4a0f707 100644
--- a/src/coreclr/src/jit/codegenxarch.cpp
+++ b/src/coreclr/src/jit/codegenxarch.cpp
@@ -3689,18 +3689,28 @@ void CodeGen::genRangeCheck(GenTree* oper)
     noway_assert(oper->OperIsBoundsCheck());
     GenTreeBoundsChk* bndsChk = oper->AsBoundsChk();
 
-    GenTree* arrIndex  = bndsChk->gtIndex;
-    GenTree* arrLen    = bndsChk->gtArrLen;
-    GenTree* arrRef    = nullptr;
-    int      lenOffset = 0;
+    GenTree* arrIndex = bndsChk->gtIndex;
+    GenTree* arrLen   = bndsChk->gtArrLen;
 
     GenTree *    src1, *src2;
     emitJumpKind jmpKind;
+    instruction  cmpKind;
 
     genConsumeRegs(arrIndex);
     genConsumeRegs(arrLen);
 
-    if (arrIndex->isContainedIntOrIImmed())
+    if (arrIndex->IsIntegralConst(0) && arrLen->isUsedFromReg())
+    {
+        // arrIndex is 0 and arrLen is in a reg. In this case
+        // we can generate
+        //      test reg, reg
+        // since arrLen is non-negative
+        src1    = arrLen;
+        src2    = arrLen;
+        jmpKind = EJ_je;
+        cmpKind = INS_test;
+    }
+    else if (arrIndex->isContainedIntOrIImmed())
     {
         // arrIndex is a contained constant.  In this case
         // we will generate one of the following
@@ -3713,6 +3723,7 @@ void CodeGen::genRangeCheck(GenTree* oper)
         src1    = arrLen;
         src2    = arrIndex;
         jmpKind = EJ_jbe;
+        cmpKind = INS_cmp;
     }
     else
     {
@@ -3721,7 +3732,7 @@ void CodeGen::genRangeCheck(GenTree* oper)
         //      cmp  [mem], immed   (if arrLen is a constant)
         //      cmp  [mem], reg     (if arrLen is in a reg)
         //      cmp  reg, immed     (if arrIndex is in a reg)
-        //      cmp  reg1, reg2     (if arraIndex is in reg1)
+        //      cmp  reg1, reg2     (if arrIndex is in reg1)
         //      cmp  reg, [mem]     (if arrLen is a memory op)
         //
         // That is only one of arrIndex or arrLen can be a memory op.
@@ -3730,6 +3741,7 @@ void CodeGen::genRangeCheck(GenTree* oper)
         src1    = arrIndex;
         src2    = arrLen;
         jmpKind = EJ_jae;
+        cmpKind = INS_cmp;
     }
 
     var_types bndsChkType = src2->TypeGet();
@@ -3741,7 +3753,7 @@ void CodeGen::genRangeCheck(GenTree* oper)
     assert(emitTypeSize(bndsChkType) >= emitTypeSize(src1->TypeGet()));
 #endif // DEBUG
 
-    GetEmitter()->emitInsBinary(INS_cmp, emitTypeSize(bndsChkType), src1, src2);
+    GetEmitter()->emitInsBinary(cmpKind, emitTypeSize(bndsChkType), src1, src2);
     genJumpToThrowHlpBlk(jmpKind, bndsChk->gtThrowKind, bndsChk->gtIndRngFailBB);
 }
 
-- 
2.7.4