Hide 'align' instruction behind jmp

src/coreclr/jit/block.cpp

@@ -1662,3 +1662,22 @@ BBswtDesc::BBswtDesc(Compiler* comp, const BBswtDesc* other)
         bbsDstTab[i] = other->bbsDstTab[i];
     }
 }
+
+//------------------------------------------------------------------------
+// unmarkLoopAlign: Unmarks the LOOP_ALIGN flag from the block and reduce the
+//                  loop alignment count.
+//
+// Arguments:
+//    compiler - Compiler instance
+//    reason - Reason to print in JITDUMP
+//
+void BasicBlock::unmarkLoopAlign(Compiler* compiler DEBUG_ARG(const char* reason))
+{
+    // Make sure we unmark and count just once.
+    if (isLoopAlign())
+    {
+        compiler->loopAlignCandidates--;
+        bbFlags &= ~BBF_LOOP_ALIGN;
+        JITDUMP("Unmarking LOOP_ALIGN from " FMT_BB ". Reason= %s.", bbNum, reason);
+    }
+}

src/coreclr/jit/block.h

@@ -552,6 +552,7 @@ enum BasicBlockFlags : unsigned __int64
     BBF_PATCHPOINT           = MAKE_BBFLAG(36), // Block is a patchpoint
     BBF_HAS_CLASS_PROFILE    = MAKE_BBFLAG(37), // BB contains a call needing a class profile
     BBF_PARTIAL_COMPILATION_PATCHPOINT  = MAKE_BBFLAG(38), // Block is a partial compilation patchpoint
+    BBF_HAS_ALIGN          = MAKE_BBFLAG(39), // BB ends with 'align' instruction
 
     // The following are sets of flags.
 
@@ -653,11 +654,19 @@ struct BasicBlock : private LIR::Range
     {
         return ((bbFlags & BBF_LOOP_HEAD) != 0);
     }
+
     bool isLoopAlign() const
     {
         return ((bbFlags & BBF_LOOP_ALIGN) != 0);
     }
 
+    void unmarkLoopAlign(Compiler* comp DEBUG_ARG(const char* reason));
+
+    bool hasAlign() const
+    {
+        return ((bbFlags & BBF_HAS_ALIGN) != 0);
+    }
+
 #ifdef DEBUG
     void     dspFlags();               // Print the flags
     unsigned dspCheapPreds();          // Print the predecessors (bbCheapPreds)

src/coreclr/jit/codegenlinear.cpp

@@ -174,6 +174,7 @@ void CodeGen::genCodeForBBlist()
 
     for (block = compiler->fgFirstBB; block != nullptr; block = block->bbNext)
     {
+
 #ifdef DEBUG
         if (compiler->verbose)
         {
@@ -782,21 +783,29 @@ void CodeGen::genCodeForBBlist()
         }
 
 #if FEATURE_LOOP_ALIGN
+        if (block->hasAlign())
+        {
+            // If this block has 'align' instruction in the end (identified by BBF_HAS_ALIGN),
+            // then need to add align instruction in the current "block".
+            //
+            // For non-adaptive alignment, add alignment instruction of size depending on the
+            // compJitAlignLoopBoundary.
+            // For adaptive alignment, alignment instruction will always be of 15 bytes for xarch
+            // and 16 bytes for arm64.
+            assert(ShouldAlignLoops());
 
-        // If next block is the first block of a loop (identified by BBF_LOOP_ALIGN),
-        // then need to add align instruction in current "block". Also mark the
-        // corresponding IG with IGF_LOOP_ALIGN to know that there will be align
-        // instructions at the end of that IG.
-        //
-        // For non-adaptive alignment, add alignment instruction of size depending on the
-        // compJitAlignLoopBoundary.
-        // For adaptive alignment, alignment instruction will always be of 15 bytes.
+            GetEmitter()->emitLoopAlignment(DEBUG_ARG1(block->bbJumpKind == BBJ_ALWAYS));
+        }
 
         if ((block->bbNext != nullptr) && (block->bbNext->isLoopAlign()))
         {
-            assert(ShouldAlignLoops());
-
-            GetEmitter()->emitLoopAlignment();
+            if (compiler->opts.compJitHideAlignBehindJmp)
+            {
+                // The current IG is the one that is just before the IG having loop start.
+                // Establish a connection of recent align instruction emitted to the loop
+                // it actually is aligning using 'idaLoopHeadPredIG'.
+                GetEmitter()->emitConnectAlignInstrWithCurIG();
+            }
         }
 #endif
 

src/coreclr/jit/compiler.cpp

@@ -2547,11 +2547,13 @@ void Compiler::compInitOptions(JitFlags* jitFlags)
 
     opts.compJitAlignLoopForJcc      = JitConfig.JitAlignLoopForJcc() == 1;
     opts.compJitAlignLoopMaxCodeSize = (unsigned short)JitConfig.JitAlignLoopMaxCodeSize();
+    opts.compJitHideAlignBehindJmp   = JitConfig.JitHideAlignBehindJmp() == 1;
 #else
     opts.compJitAlignLoopAdaptive       = true;
     opts.compJitAlignLoopBoundary       = DEFAULT_ALIGN_LOOP_BOUNDARY;
     opts.compJitAlignLoopMinBlockWeight = DEFAULT_ALIGN_LOOP_MIN_BLOCK_WEIGHT;
     opts.compJitAlignLoopMaxCodeSize    = DEFAULT_MAX_LOOPSIZE_FOR_ALIGN;
+    opts.compJitHideAlignBehindJmp      = true;
 #endif
 
 #ifdef TARGET_XARCH
@@ -5152,6 +5154,11 @@ void Compiler::compCompile(void** methodCodePtr, uint32_t* methodCodeSize, JitFl
     fgDebugCheckLinks();
 #endif
 
+#if FEATURE_LOOP_ALIGN
+    // Place loop alignment instructions
+    DoPhase(this, PHASE_ALIGN_LOOPS, &Compiler::placeLoopAlignInstructions);
+#endif
+
     // Generate code
     codeGen->genGenerateCode(methodCodePtr, methodCodeSize);
 
@@ -5208,6 +5215,82 @@ void Compiler::compCompile(void** methodCodePtr, uint32_t* methodCodeSize, JitFl
 #endif // FUNC_INFO_LOGGING
 }
 
+#if FEATURE_LOOP_ALIGN
+
+//------------------------------------------------------------------------
+// placeLoopAlignInstructions: Iterate over all the blocks and determine
+//      the best position to place the 'align' instruction. Inserting 'align'
+//      instructions after an unconditional branch is preferred over inserting
+//      in the block before the loop. In case there are multiple blocks
+//      having 'jmp', the one that has lower weight is preferred.
+//      If the block having 'jmp' is hotter than the block before the loop,
+//      the align will still be placed after 'jmp' because the processor should
+//      be smart enough to not fetch extra instruction beyond jmp.
+//
+void Compiler::placeLoopAlignInstructions()
+{
+    if (loopAlignCandidates == 0)
+    {
+        return;
+    }
+
+    int loopsToProcess = loopAlignCandidates;
+
+    // Add align only if there were any loops that needed alignment
+    weight_t    minBlockSoFar = BB_MAX_WEIGHT;
+    BasicBlock* bbHavingAlign = nullptr;
+    for (BasicBlock* const block : Blocks())
+    {
+        if ((block == fgFirstBB) && block->isLoopAlign())
+        {
+            // Adding align instruction in prolog is not supported
+            // hence skip the align block if it is the first block.
+            loopsToProcess--;
+            continue;
+        }
+
+        // If there is a unconditional jump
+        if (opts.compJitHideAlignBehindJmp && (block->bbJumpKind == BBJ_ALWAYS))
+        {
+            if (block->bbWeight < minBlockSoFar)
+            {
+                minBlockSoFar = block->bbWeight;
+                bbHavingAlign = block;
+                JITDUMP(FMT_BB ", bbWeight=" FMT_WT " ends with unconditional 'jmp' \n", block->bbNum, block->bbWeight);
+            }
+        }
+
+        if ((block->bbNext != nullptr) && (block->bbNext->isLoopAlign()))
+        {
+            // If jmp was not found, then block before the loop start is where align instruction will be added.
+            if (bbHavingAlign == nullptr)
+            {
+                bbHavingAlign = block;
+                JITDUMP("Marking " FMT_BB " before the loop with BBF_HAS_ALIGN for loop at " FMT_BB "\n", block->bbNum,
+                        block->bbNext->bbNum);
+            }
+            else
+            {
+                JITDUMP("Marking " FMT_BB " that ends with unconditional jump with BBF_HAS_ALIGN for loop at " FMT_BB
+                        "\n",
+                        bbHavingAlign->bbNum, block->bbNext->bbNum);
+            }
+
+            bbHavingAlign->bbFlags |= BBF_HAS_ALIGN;
+            minBlockSoFar = BB_MAX_WEIGHT;
+            bbHavingAlign = nullptr;
+
+            if (--loopsToProcess == 0)
+            {
+                break;
+            }
+        }
+    }
+
+    assert(loopsToProcess == 0);
+}
+#endif
+
 //------------------------------------------------------------------------
 // generatePatchpointInfo: allocate and fill in patchpoint info data,
 //    and report it to the VM

src/coreclr/jit/compiler.h

@@ -3666,6 +3666,7 @@ class Compiler
 #endif
 
     BasicBlock* bbNewBasicBlock(BBjumpKinds jumpKind);
+    void placeLoopAlignInstructions();
 
     /*
     XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
@@ -6871,13 +6872,13 @@ class Compiler
     bool fgHasLoops;        // True if this method has any loops, set in fgComputeReachability
 
 public:
-    LoopDsc*      optLoopTable; // loop descriptor table
-    unsigned char optLoopCount; // number of tracked loops
+    LoopDsc*      optLoopTable;        // loop descriptor table
+    unsigned char optLoopCount;        // number of tracked loops
+    unsigned char loopAlignCandidates; // number of loops identified for alignment
 
 #ifdef DEBUG
-    unsigned char loopAlignCandidates; // number of loops identified for alignment
-    unsigned char loopsAligned;        // number of loops actually aligned
-#endif                                 // DEBUG
+    unsigned char loopsAligned; // number of loops actually aligned
+#endif                          // DEBUG
 
     bool optRecordLoop(BasicBlock*   head,
                        BasicBlock*   top,
@@ -9687,6 +9688,9 @@ XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
         // If set, perform adaptive loop alignment that limits number of padding based on loop size.
         bool compJitAlignLoopAdaptive;
 
+        // If set, tries to hide alignment instructions behind unconditional jumps.
+        bool compJitHideAlignBehindJmp;
+
 #ifdef LATE_DISASM
         bool doLateDisasm; // Run the late disassembler
 #endif                     // LATE_DISASM

src/coreclr/jit/compphases.h

@@ -87,6 +87,7 @@ CompPhaseNameMacro(PHASE_INSERT_GC_POLLS,        "Insert GC Polls",
 CompPhaseNameMacro(PHASE_DETERMINE_FIRST_COLD_BLOCK, "Determine first cold block", "COLD-BLK", false, -1, true)
 CompPhaseNameMacro(PHASE_RATIONALIZE,            "Rationalize IR",                 "RAT",      false, -1, false)
 CompPhaseNameMacro(PHASE_SIMPLE_LOWERING,        "Do 'simple' lowering",           "SMP-LWR",  false, -1, false)
+CompPhaseNameMacro(PHASE_ALIGN_LOOPS,            "Place 'align' instructions",     "LOOP-ALIGN",  false, -1, false)
 
 CompPhaseNameMacro(PHASE_LCLVARLIVENESS,         "Local var liveness",             "LIVENESS", true, -1, false)
 CompPhaseNameMacro(PHASE_LCLVARLIVENESS_INIT,    "Local var liveness init",        "LIV-INIT", false, PHASE_LCLVARLIVENESS, false)