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Change heuristic to check the increase in # phi entries
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@Chengjunp
Chengjunp committed Sep 5, 2024
1 parent bebecef commit f86d2de
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Showing 2 changed files with 81 additions and 35 deletions.
33 changes: 17 additions & 16 deletions llvm/lib/Transforms/Utils/Local.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -112,10 +112,11 @@ static cl::opt<unsigned> PHICSENumPHISmallSize(
"When the basic block contains not more than this number of PHI nodes, "
"perform a (faster!) exhaustive search instead of set-driven one."));

static cl::opt<unsigned> MaxPhiEntriesAfterRemovingEmptyBlock(
"max-phi-entries-after-removing-empty-block", cl::init(100), cl::Hidden,
cl::desc("Stop removing an empty block if removing it will make a PHI have "
"more than this number of incoming entries."));
static cl::opt<unsigned> MaxPhiEntriesIncreaseAfterRemovingEmptyBlock(
"max-phi-entries-increase-after-removing-empty-block", cl::init(1000),
cl::Hidden,
cl::desc("Stop removing an empty block if removing it will introduce more "
"than this number of phi entries in its successor"));

// Max recursion depth for collectBitParts used when detecting bswap and
// bitreverse idioms.
Expand Down Expand Up @@ -1048,29 +1049,29 @@ CanRedirectPredsOfEmptyBBToSucc(BasicBlock *BB, BasicBlock *Succ,
// Check whether removing BB will make the phis in its Succ have too
// many incoming entries. This function does not check whether BB is foldable
// or not.
static bool introduceTooComplexPhi(BasicBlock *BB, BasicBlock *Succ) {
static bool introduceTooManyPhiEntries(BasicBlock *BB, BasicBlock *Succ) {
// If BB only has one predecessor, then removing it will not introduce more
// incoming edges for phis.
if (BB->hasNPredecessors(1))
return false;
int NumPreds = pred_size(BB);
unsigned NumPreds = pred_size(BB);
unsigned NumChangedPhi = 0;
for (auto &Phi : Succ->phis()) {
// If the incoming value is a phi and the phi is defined in BB,
// then removing BB will not increase the total phi entries of the ir.
if (PHINode *IncomingPhi =
dyn_cast<PHINode>(Phi.getIncomingValueForBlock(BB)))
if (IncomingPhi->getParent() == BB)
continue;
// Otherwise, we need to add (NumPreds - 1) entries to the phi node.
// If removing BB makes the phi have more than
// MaxPhiEntriesAfterRemovingEmptyBlock incoming values, then it will be
// considered as introducing too complex phi to the ir.
// The default threshold is 100.
if ((NumPreds - 1) + Phi.getNumIncomingValues() >
MaxPhiEntriesAfterRemovingEmptyBlock)
return true;
// Otherwise, we need to add entries to the phi
NumChangedPhi++;
}
return false;
// For every phi that needs to be changed, (NumPreds - 1) new entries will be
// added. If the total increase in phi entries exceeds
// MaxPhiEntriesIncreaseAfterRemovingEmptyBlock, it will be considered as
// introducing too many new phi entries.
return (NumPreds - 1) * NumChangedPhi >
MaxPhiEntriesIncreaseAfterRemovingEmptyBlock;
}

/// Replace a value flowing from a block to a phi with
Expand Down Expand Up @@ -1172,7 +1173,7 @@ bool llvm::TryToSimplifyUncondBranchFromEmptyBlock(BasicBlock *BB,
BBKillable ||
CanRedirectPredsOfEmptyBBToSucc(BB, Succ, BBPreds, SuccPreds, CommonPred);

if ((!BBKillable && !BBPhisMergeable) || introduceTooComplexPhi(BB, Succ))
if ((!BBKillable && !BBPhisMergeable) || introduceTooManyPhiEntries(BB, Succ))
return false;

// Check to see if merging these blocks/phis would cause conflicts for any of
Expand Down
83 changes: 64 additions & 19 deletions llvm/test/Transforms/SimplifyCFG/avoid-complex-phi.ll
View file
Original file line number Diff line number Diff line change
@@ -1,23 +1,59 @@
; RUN: opt < %s -passes=simplifycfg -S | FileCheck --check-prefixes=CHECK-100 %s
; RUN: opt < %s -max-phi-entries-after-removing-empty-block=101 -passes=simplifycfg -S | FileCheck --check-prefixes=CHECK-101 %s
; CHECK-100: %x = phi i16 {{((\[ [0-2], %BB[0-9]+ \], ){51})\[ [0-2], %BB[0-9]+ \]}}
; CHECK-101: %x = phi i16 {{((\[ [0-2], %BB[0-9]+ \], ){100})\[ [0-2], %BB[0-9]+ \]}}
; RUN: opt < %s -passes=simplifycfg -S | FileCheck --check-prefixes=CHECK-1000 %s
; RUN: opt < %s -max-phi-entries-increase-after-removing-empty-block=989 -passes=simplifycfg -S | FileCheck --check-prefixes=CHECK-989 %s
; RUN: opt < %s -max-phi-entries-increase-after-removing-empty-block=489 -passes=simplifycfg -S | FileCheck --check-prefixes=CHECK-489 %s

; This test has the following CFG:
; 1. entry has a switch to 100 blocks: BB1 - BB100
; 2. For BB1 to BB50, it branch to BB101 and BB103
; 3. For BB51 to BB100, it branch to BB102 and BB103
; 2. For BB1 to BB50, it branches to BB101 and BB103
; 3. For BB51 to BB100, it branches to BB102 and BB103
; 4. BB101, BB102, BB103 branch to Merge unconditionally
; 5. Merge returns value %x which is a phi having incoming blocks BB101, BB102, BB103
; 5. Merge has 10 phis(x1 - x10).
;
; In the first test, we are going to check that the simplifycfg will not introduce a phi with more than
; 100 incoming values. So the simplifycfg can remove only one of BB101 and BB102. Otherwise, the
; phi of %x will have at least 50 + 50 + 1 = 101 incoming blocks (BB1 - BB100 and BB103).
;
; In the second test, the threshold is changed to 101. Then both BB101 and BB102 can be removed, and
; %x will have 101 incoming entries.
; If we remove BB103, it will increase the number of phi entries by (100 - 1) * 10 = 990.
; If we remove BB101 / BB102, it will increase the number of phi entries by (50 - 1) * 10 = 490.
;
define i16 @example(i32 %a, ptr %array) {
; By default, in SimplifyCFG, we will not remove a block if it will increase more than 1000 phi entries.
; In the first test, BB103 will be removed, and every phi will have 102(3 + 100 - 1) phi entries.
; In the second test, we set max-phi-entries-increase-after-removing-empty-block to be 989, then BB103 should not be removed,
; but BB101 and BB102 can be removed.
; In the third test, we set max-phi-entries-increase-after-removing-empty-block to be 489, then no BB can be removed.

; CHECK-1000: %x1 = phi i16 {{((\[ [0-9], %BB[0-9]+ \], ){101})\[ [0-9], %BB[0-9]+ \]}}
; CHECK-1000: %x2 = phi i16 {{((\[ [0-9], %BB[0-9]+ \], ){101})\[ [0-9], %BB[0-9]+ \]}}
; CHECK-1000: %x3 = phi i16 {{((\[ [0-9], %BB[0-9]+ \], ){101})\[ [0-9], %BB[0-9]+ \]}}
; CHECK-1000: %x4 = phi i16 {{((\[ [0-9], %BB[0-9]+ \], ){101})\[ [0-9], %BB[0-9]+ \]}}
; CHECK-1000: %x5 = phi i16 {{((\[ [0-9], %BB[0-9]+ \], ){101})\[ [0-9], %BB[0-9]+ \]}}
; CHECK-1000: %x6 = phi i16 {{((\[ [0-9], %BB[0-9]+ \], ){101})\[ [0-9], %BB[0-9]+ \]}}
; CHECK-1000: %x7 = phi i16 {{((\[ [0-9], %BB[0-9]+ \], ){101})\[ [0-9], %BB[0-9]+ \]}}
; CHECK-1000: %x8 = phi i16 {{((\[ [0-9], %BB[0-9]+ \], ){101})\[ [0-9], %BB[0-9]+ \]}}
; CHECK-1000: %x9 = phi i16 {{((\[ [0-9], %BB[0-9]+ \], ){101})\[ [0-9], %BB[0-9]+ \]}}
; CHECK-1000: %x10 = phi i16 {{((\[ [0-9], %BB[0-9]+ \], ){101})\[ [0-9], %BB[0-9]+ \]}}

; CHECK-989: %x1 = phi i16 {{((\[ [0-9], %BB[0-9]+ \], ){100})\[ [0-9], %BB[0-9]+ \]}}
; CHECK-989: %x2 = phi i16 {{((\[ [0-9], %BB[0-9]+ \], ){100})\[ [0-9], %BB[0-9]+ \]}}
; CHECK-989: %x3 = phi i16 {{((\[ [0-9], %BB[0-9]+ \], ){100})\[ [0-9], %BB[0-9]+ \]}}
; CHECK-989: %x4 = phi i16 {{((\[ [0-9], %BB[0-9]+ \], ){100})\[ [0-9], %BB[0-9]+ \]}}
; CHECK-989: %x5 = phi i16 {{((\[ [0-9], %BB[0-9]+ \], ){100})\[ [0-9], %BB[0-9]+ \]}}
; CHECK-989: %x6 = phi i16 {{((\[ [0-9], %BB[0-9]+ \], ){100})\[ [0-9], %BB[0-9]+ \]}}
; CHECK-989: %x7 = phi i16 {{((\[ [0-9], %BB[0-9]+ \], ){100})\[ [0-9], %BB[0-9]+ \]}}
; CHECK-989: %x8 = phi i16 {{((\[ [0-9], %BB[0-9]+ \], ){100})\[ [0-9], %BB[0-9]+ \]}}
; CHECK-989: %x9 = phi i16 {{((\[ [0-9], %BB[0-9]+ \], ){100})\[ [0-9], %BB[0-9]+ \]}}
; CHECK-989: %x10 = phi i16 {{((\[ [0-9], %BB[0-9]+ \], ){100})\[ [0-9], %BB[0-9]+ \]}}

; CHECK-489: %x1 = phi i16 {{((\[ [0-9], %BB[0-9]+ \], ){2})\[ [0-9], %BB[0-9]+ \]}}
; CHECK-489: %x2 = phi i16 {{((\[ [0-9], %BB[0-9]+ \], ){2})\[ [0-9], %BB[0-9]+ \]}}
; CHECK-489: %x3 = phi i16 {{((\[ [0-9], %BB[0-9]+ \], ){2})\[ [0-9], %BB[0-9]+ \]}}
; CHECK-489: %x4 = phi i16 {{((\[ [0-9], %BB[0-9]+ \], ){2})\[ [0-9], %BB[0-9]+ \]}}
; CHECK-489: %x5 = phi i16 {{((\[ [0-9], %BB[0-9]+ \], ){2})\[ [0-9], %BB[0-9]+ \]}}
; CHECK-489: %x6 = phi i16 {{((\[ [0-9], %BB[0-9]+ \], ){2})\[ [0-9], %BB[0-9]+ \]}}
; CHECK-489: %x7 = phi i16 {{((\[ [0-9], %BB[0-9]+ \], ){2})\[ [0-9], %BB[0-9]+ \]}}
; CHECK-489: %x8 = phi i16 {{((\[ [0-9], %BB[0-9]+ \], ){2})\[ [0-9], %BB[0-9]+ \]}}
; CHECK-489: %x9 = phi i16 {{((\[ [0-9], %BB[0-9]+ \], ){2})\[ [0-9], %BB[0-9]+ \]}}
; CHECK-489: %x10 = phi i16 {{((\[ [0-9], %BB[0-9]+ \], ){2})\[ [0-9], %BB[0-9]+ \]}}


;
define void @example(i32 %a, ptr %array) {
entry:
switch i32 %a, label %BB1 [
i32 1, label %BB1
Expand Down Expand Up @@ -722,16 +758,25 @@ BB100: ; preds = %entry
%cmp100 = icmp eq i32 %val100, 100
br i1 %cmp100, label %BB102, label %BB103

BB101: ; preds = %BB50, %BB49, %BB48, %BB47, %BB46, %BB45, %BB44, %BB43, %BB42, %BB41, %BB40, %BB39, %BB38, %BB37, %BB36, %BB35, %BB34, %BB33, %BB32, %BB31, %BB30, %BB29, %BB28, %BB27, %BB26, %BB25, %BB24, %BB23, %BB22, %BB21, %BB20, %BB19, %BB18, %BB17, %BB16, %BB15, %BB14, %BB13, %BB12, %BB11, %BB10, %BB9, %BB8, %BB7, %BB6, %BB5, %BB4, %BB3, %BB2, %BB1
BB103: ; preds = %BB100, %BB99, %BB98, %BB97, %BB96, %BB95, %BB94, %BB93, %BB92, %BB91, %BB90, %BB89, %BB88, %BB87, %BB86, %BB85, %BB84, %BB83, %BB82, %BB81, %BB80, %BB79, %BB78, %BB77, %BB76, %BB75, %BB74, %BB73, %BB72, %BB71, %BB70, %BB69, %BB68, %BB67, %BB66, %BB65, %BB64, %BB63, %BB62, %BB61, %BB60, %BB59, %BB58, %BB57, %BB56, %BB55, %BB54, %BB53, %BB52, %BB51, %BB50, %BB49, %BB48, %BB47, %BB46, %BB45, %BB44, %BB43, %BB42, %BB41, %BB40, %BB39, %BB38, %BB37, %BB36, %BB35, %BB34, %BB33, %BB32, %BB31, %BB30, %BB29, %BB28, %BB27, %BB26, %BB25, %BB24, %BB23, %BB22, %BB21, %BB20, %BB19, %BB18, %BB17, %BB16, %BB15, %BB14, %BB13, %BB12, %BB11, %BB10, %BB9, %BB8, %BB7, %BB6, %BB5, %BB4, %BB3, %BB2, %BB1
br label %Merge

BB102: ; preds = %BB100, %BB99, %BB98, %BB97, %BB96, %BB95, %BB94, %BB93, %BB92, %BB91, %BB90, %BB89, %BB88, %BB87, %BB86, %BB85, %BB84, %BB83, %BB82, %BB81, %BB80, %BB79, %BB78, %BB77, %BB76, %BB75, %BB74, %BB73, %BB72, %BB71, %BB70, %BB69, %BB68, %BB67, %BB66, %BB65, %BB64, %BB63, %BB62, %BB61, %BB60, %BB59, %BB58, %BB57, %BB56, %BB55, %BB54, %BB53, %BB52, %BB51
BB101: ; preds = %BB50, %BB49, %BB48, %BB47, %BB46, %BB45, %BB44, %BB43, %BB42, %BB41, %BB40, %BB39, %BB38, %BB37, %BB36, %BB35, %BB34, %BB33, %BB32, %BB31, %BB30, %BB29, %BB28, %BB27, %BB26, %BB25, %BB24, %BB23, %BB22, %BB21, %BB20, %BB19, %BB18, %BB17, %BB16, %BB15, %BB14, %BB13, %BB12, %BB11, %BB10, %BB9, %BB8, %BB7, %BB6, %BB5, %BB4, %BB3, %BB2, %BB1
br label %Merge

BB103: ; preds = %BB100, %BB99, %BB98, %BB97, %BB96, %BB95, %BB94, %BB93, %BB92, %BB91, %BB90, %BB89, %BB88, %BB87, %BB86, %BB85, %BB84, %BB83, %BB82, %BB81, %BB80, %BB79, %BB78, %BB77, %BB76, %BB75, %BB74, %BB73, %BB72, %BB71, %BB70, %BB69, %BB68, %BB67, %BB66, %BB65, %BB64, %BB63, %BB62, %BB61, %BB60, %BB59, %BB58, %BB57, %BB56, %BB55, %BB54, %BB53, %BB52, %BB51, %BB50, %BB49, %BB48, %BB47, %BB46, %BB45, %BB44, %BB43, %BB42, %BB41, %BB40, %BB39, %BB38, %BB37, %BB36, %BB35, %BB34, %BB33, %BB32, %BB31, %BB30, %BB29, %BB28, %BB27, %BB26, %BB25, %BB24, %BB23, %BB22, %BB21, %BB20, %BB19, %BB18, %BB17, %BB16, %BB15, %BB14, %BB13, %BB12, %BB11, %BB10, %BB9, %BB8, %BB7, %BB6, %BB5, %BB4, %BB3, %BB2, %BB1
BB102: ; preds = %BB100, %BB99, %BB98, %BB97, %BB96, %BB95, %BB94, %BB93, %BB92, %BB91, %BB90, %BB89, %BB88, %BB87, %BB86, %BB85, %BB84, %BB83, %BB82, %BB81, %BB80, %BB79, %BB78, %BB77, %BB76, %BB75, %BB74, %BB73, %BB72, %BB71, %BB70, %BB69, %BB68, %BB67, %BB66, %BB65, %BB64, %BB63, %BB62, %BB61, %BB60, %BB59, %BB58, %BB57, %BB56, %BB55, %BB54, %BB53, %BB52, %BB51
br label %Merge

Merge: ; preds = %BB103, %BB102, %BB101
%x = phi i16 [ 0, %BB101 ], [ 1, %BB102 ], [ 2, %BB103 ]
ret i16 %x
%x1 = phi i16 [ 1, %BB103 ], [ 0, %BB101 ], [ 2, %BB102 ]
%x2 = phi i16 [ 2 , %BB103 ], [ 0, %BB101 ], [ 2, %BB102 ]
%x3 = phi i16 [ 3 , %BB103 ], [ 0, %BB101 ], [ 2, %BB102 ]
%x4 = phi i16 [ 4 , %BB103 ], [ 0, %BB101 ], [ 2, %BB102 ]
%x5 = phi i16 [ 5 , %BB103 ], [ 0, %BB101 ], [ 2, %BB102 ]
%x6 = phi i16 [ 6 , %BB103 ], [ 0, %BB101 ], [ 2, %BB102 ]
%x7 = phi i16 [ 7 , %BB103 ], [ 0, %BB101 ], [ 2, %BB102 ]
%x8 = phi i16 [ 8 , %BB103 ], [ 0, %BB101 ], [ 2, %BB102 ]
%x9 = phi i16 [ 9 , %BB103 ], [ 0, %BB101 ], [ 2, %BB102 ]
%x10 = phi i16 [ 0, %BB103 ], [ 0, %BB101 ], [ 2, %BB102 ]
ret void
}

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