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Loop alignment: Handle blocks added in loop as part of split edges of LSRA #55047

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Jul 8, 2021
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Loop alignment: Handle blocks added in loop as part of split edges of LSRA #55047

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2565e4d
Handle blocks added in loop as part of split edges of LSRA
kunalspathak Jul 1, 2021
5d777fa
Retain LOOP_ALIGN flag of loops whose start are same
kunalspathak Jul 2, 2021
aae9dcd
jit format
kunalspathak Jul 2, 2021
cd2004c
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kunalspathak Jul 8, 2021
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114 changes: 84 additions & 30 deletions src/coreclr/jit/emit.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1023,9 +1023,9 @@ void emitter::emitBegFN(bool hasFramePtr
emitIGbuffSize = 0;

#if FEATURE_LOOP_ALIGN
emitLastAlignedIgNum = 0;
emitLastInnerLoopStartIgNum = 0;
emitLastInnerLoopEndIgNum = 0;
emitLastAlignedIgNum = 0;
emitLastLoopStart = 0;
emitLastLoopEnd = 0;
#endif

/* Record stack frame info (the temp size is just an estimate) */
Expand Down Expand Up @@ -4820,58 +4820,112 @@ unsigned emitter::getLoopSize(insGroup* igLoopHeader, unsigned maxLoopSize DEBUG
// if currIG has back-edge to dstIG.
//
// Notes:
// If the current loop encloses a loop that is already marked as align, then remove
// the alignment flag present on IG before dstIG.
// Despite we align only inner most loop, we might see intersected loops because of control flow
// re-arrangement like adding a split edge in LSRA.
//
// If there is an intersection of current loop with last loop that is already marked as align,
// then *do not align* one of the loop that completely encloses the other one. Or if they both intersect,
// then *do not align* either of them because since the flow is complicated enough that aligning one of them
// will not improve the performance.
//
void emitter::emitSetLoopBackEdge(BasicBlock* loopTopBlock)
{
insGroup* dstIG = (insGroup*)loopTopBlock->bbEmitCookie;
insGroup* dstIG = (insGroup*)loopTopBlock->bbEmitCookie;
bool noAlignCurrentLoop = false;
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bool noAlignLastLoop = false;

// With (dstIG != nullptr), ensure that only back edges are tracked.
// If there is forward jump, dstIG is not yet generated.
//
// We don't rely on (block->bbJumpDest->bbNum <= block->bbNum) because the basic
// block numbering is not guaranteed to be sequential.

if ((dstIG != nullptr) && (dstIG->igNum <= emitCurIG->igNum))
{
unsigned currLoopStart = dstIG->igNum;
unsigned currLoopEnd = emitCurIG->igNum;

// Only mark back-edge if current loop starts after the last inner loop ended.
if (emitLastInnerLoopEndIgNum < currLoopStart)
if (emitLastLoopEnd < currLoopStart)
{
emitCurIG->igLoopBackEdge = dstIG;

JITDUMP("** IG%02u jumps back to IG%02u forming a loop.\n", currLoopEnd, currLoopStart);

emitLastInnerLoopStartIgNum = currLoopStart;
emitLastInnerLoopEndIgNum = currLoopEnd;
emitLastLoopStart = currLoopStart;
emitLastLoopEnd = currLoopEnd;
}
else if (currLoopStart == emitLastLoopStart)
{
// Note: If current and last loop starts at same point,
// retain the alignment flag of the smaller loop.
// |
// .---->|<----.
// last | | |
// loop | | | current
// .---->| | loop
// | |
// |-----.
//
}
else if ((currLoopStart < emitLastLoopStart) && (emitLastLoopEnd < currLoopEnd))
{
// if current loop completely encloses last loop,
// then current loop should not be aligned.
noAlignCurrentLoop = true;
}
else if ((emitLastLoopStart < currLoopStart) && (currLoopEnd < emitLastLoopEnd))
{
// if last loop completely encloses current loop,
// then last loop should not be aligned.
noAlignLastLoop = true;
}
else
{
// The loops intersect and should not align either of the loops
noAlignLastLoop = true;
noAlignCurrentLoop = true;
}
// Otherwise, mark the dstIG->prevIG as no alignment needed.
//
// Note: If current loop's back-edge target is same as emitLastInnerLoopStartIgNum,
// retain the alignment flag of dstIG->prevIG so the loop
// (emitLastInnerLoopStartIgNum ~ emitLastInnerLoopEndIgNum) is still aligned.
else if (emitLastInnerLoopStartIgNum != currLoopStart)
{
// Find the IG before dstIG...
instrDescAlign* alignInstr = emitAlignList;
while ((alignInstr != nullptr) && (alignInstr->idaIG->igNext != dstIG))
{
alignInstr = alignInstr->idaNext;
}

// ...and clear the IGF_LOOP_ALIGN flag
if (alignInstr != nullptr)
if (noAlignLastLoop || noAlignCurrentLoop)
{
instrDescAlign* alignInstr = emitAlignList;
bool markedLastLoop = !noAlignLastLoop;
bool markedCurrLoop = !noAlignCurrentLoop;
while ((alignInstr != nullptr))
{
assert(alignInstr->idaIG->igNext == dstIG);
alignInstr->idaIG->igFlags &= ~IGF_LOOP_ALIGN;
// Find the IG before current loop and clear the IGF_LOOP_ALIGN flag
if (noAlignCurrentLoop && (alignInstr->idaIG->igNext == dstIG))
{
assert(!markedCurrLoop);
alignInstr->idaIG->igFlags &= ~IGF_LOOP_ALIGN;
markedCurrLoop = true;
JITDUMP("** Skip alignment for current loop IG%02u ~ IG%02u because it encloses an aligned loop "
"IG%02u ~ IG%02u.\n",
currLoopStart, currLoopEnd, emitLastLoopStart, emitLastLoopEnd);
}

// Find the IG before the last loop and clear the IGF_LOOP_ALIGN flag
if (noAlignLastLoop && (alignInstr->idaIG->igNext != nullptr) &&
(alignInstr->idaIG->igNext->igNum == emitLastLoopStart))
{
assert(!markedLastLoop);
assert(alignInstr->idaIG->isLoopAlign());
alignInstr->idaIG->igFlags &= ~IGF_LOOP_ALIGN;
markedLastLoop = true;
JITDUMP("** Skip alignment for aligned loop IG%02u ~ IG%02u because it encloses the current loop "
"IG%02u ~ IG%02u.\n",
emitLastLoopStart, emitLastLoopEnd, currLoopStart, currLoopEnd);
}

if (markedLastLoop && markedCurrLoop)
{
break;
}

alignInstr = alignInstr->idaNext;
}

JITDUMP(
"** Skip alignment for loop IG%02u ~ IG%02u, because it encloses an aligned loop IG%02u ~ IG%02u.\n",
currLoopStart, currLoopEnd, emitLastInnerLoopStartIgNum, emitLastInnerLoopEndIgNum);
assert(markedLastLoop && markedCurrLoop);
}
}
}
Expand Down
12 changes: 6 additions & 6 deletions src/coreclr/jit/emit.h
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Original file line number Diff line number Diff line change
Expand Up @@ -1762,12 +1762,12 @@ class emitter
void emitJumpDistBind(); // Bind all the local jumps in method

#if FEATURE_LOOP_ALIGN
instrDescAlign* emitCurIGAlignList; // list of align instructions in current IG
unsigned emitLastInnerLoopStartIgNum; // Start IG of last inner loop
unsigned emitLastInnerLoopEndIgNum; // End IG of last inner loop
unsigned emitLastAlignedIgNum; // last IG that has align instruction
instrDescAlign* emitAlignList; // list of local align instructions in method
instrDescAlign* emitAlignLast; // last align instruction in method
instrDescAlign* emitCurIGAlignList; // list of align instructions in current IG
unsigned emitLastLoopStart; // Start IG of last inner loop
unsigned emitLastLoopEnd; // End IG of last inner loop
unsigned emitLastAlignedIgNum; // last IG that has align instruction
instrDescAlign* emitAlignList; // list of local align instructions in method
instrDescAlign* emitAlignLast; // last align instruction in method
unsigned getLoopSize(insGroup* igLoopHeader,
unsigned maxLoopSize DEBUG_ARG(bool isAlignAdjusted)); // Get the smallest loop size
void emitLoopAlignment();
Expand Down