[llvm] [HashRecognize] Rewrite arePHIsIntertwined (PR #144878)
Ramkumar Ramachandra via llvm-commits
llvm-commits at lists.llvm.org
Mon Jun 30 08:11:51 PDT 2025
https://github.com/artagnon updated https://github.com/llvm/llvm-project/pull/144878
>From 8467b98d789f6dc1790d7d345b491ad600459501 Mon Sep 17 00:00:00 2001
From: Ramkumar Ramachandra <ramkumar.ramachandra at codasip.com>
Date: Thu, 19 Jun 2025 11:29:47 +0100
Subject: [PATCH 1/2] [HashRecognize] Pre-commit tests for PHIsIntertwined
---
.../HashRecognize/cyclic-redundancy-check.ll | 58 ++++++++++++++++++-
1 file changed, 56 insertions(+), 2 deletions(-)
diff --git a/llvm/test/Analysis/HashRecognize/cyclic-redundancy-check.ll b/llvm/test/Analysis/HashRecognize/cyclic-redundancy-check.ll
index 0366684a13b54..1fa76cb23173b 100644
--- a/llvm/test/Analysis/HashRecognize/cyclic-redundancy-check.ll
+++ b/llvm/test/Analysis/HashRecognize/cyclic-redundancy-check.ll
@@ -144,6 +144,34 @@ exit: ; preds = %loop
ret i16 %crc.next
}
+define i8 @crc8.le.tc16(i16 %msg, i8 %checksum) {
+; CHECK-LABEL: 'crc8.le.tc16'
+; CHECK-NEXT: Did not find a hash algorithm
+; CHECK-NEXT: Reason: Simple recurrence doesn't use conditional recurrence with XOR
+;
+entry:
+ br label %loop
+
+loop: ; preds = %loop, %entry
+ %iv = phi i8 [ 0, %entry ], [ %iv.next, %loop ]
+ %crc = phi i8 [ %checksum, %entry ], [ %crc.next, %loop ]
+ %data = phi i16 [ %msg, %entry ], [ %data.next, %loop ]
+ %data.trunc = trunc i16 %data to i8
+ %xor.crc.data = xor i8 %crc, %data.trunc
+ %and.crc.data = and i8 %xor.crc.data, 1
+ %data.next = lshr i16 %data, 1
+ %check.sb = icmp eq i8 %and.crc.data, 0
+ %crc.lshr = lshr i8 %crc, 1
+ %crc.xor = xor i8 %crc.lshr, 29
+ %crc.next = select i1 %check.sb, i8 %crc.lshr, i8 %crc.xor
+ %iv.next = add nuw nsw i8 %iv, 1
+ %exit.cond = icmp samesign ult i8 %iv, 15
+ br i1 %exit.cond, label %loop, label %exit
+
+exit: ; preds = %loop
+ ret i8 %crc.next
+}
+
define i16 @crc16.be.tc8.crc.init.li(i16 %checksum, i8 %msg) {
; CHECK-LABEL: 'crc16.be.tc8.crc.init.li'
; CHECK-NEXT: Found big-endian CRC-16 loop with trip count 8
@@ -610,9 +638,8 @@ loop: ; preds = %loop, %entry
%iv = phi i8 [ 0, %entry ], [ %iv.next, %loop ]
%data = phi i8 [ %msg, %entry ], [ %data.next, %loop ]
%crc = phi i16 [ %checksum, %entry ], [ %crc.next, %loop ]
- %crc.lshr = lshr i16 %crc, 8
%data.ext = zext i8 %data to i16
- %xor.crc.data = xor i16 %crc.lshr, %data.ext
+ %xor.crc.data = xor i16 %crc, %data.ext
%check.sb = icmp samesign ult i16 %xor.crc.data, 128
%crc.shl = shl i16 %crc, 1
%crc.xor = xor i16 %crc.shl, 258
@@ -838,6 +865,33 @@ exit: ; preds = %loop
ret i16 %crc.next
}
+define i16 @not.crc.bad.cast(i8 %msg, i16 %checksum) {
+; CHECK-LABEL: 'not.crc.bad.cast'
+; CHECK-NEXT: Did not find a hash algorithm
+; CHECK-NEXT: Reason: Simple recurrence doesn't use conditional recurrence with XOR
+;
+entry:
+ br label %loop
+
+loop: ; preds = %loop, %entry
+ %iv = phi i8 [ 0, %entry ], [ %iv.next, %loop ]
+ %data = phi i8 [ %msg, %entry ], [ %data.next, %loop ]
+ %crc = phi i16 [ %checksum, %entry ], [ %crc.next, %loop ]
+ %data.ext = zext i8 %data to i16
+ %xor.crc.data = xor i16 %crc, %data.ext
+ %check.sb = icmp slt i16 %xor.crc.data, 0
+ %crc.shl = shl i16 %crc, 1
+ %crc.xor = xor i16 %crc.shl, 29
+ %crc.next = select i1 %check.sb, i16 %crc.shl, i16 %crc.xor
+ %data.next = shl i8 %data, 1
+ %iv.next = add nuw nsw i8 %iv, 1
+ %exit.cond = icmp samesign ult i8 %iv, 7
+ br i1 %exit.cond, label %loop, label %exit
+
+exit: ; preds = %loop
+ ret i16 %crc.next
+}
+
define i32 @not.crc.dead.msg.bad.use(i32 %checksum, i32 %msg) {
; CHECK-LABEL: 'not.crc.dead.msg.bad.use'
; CHECK-NEXT: Did not find a hash algorithm
>From 22cf537d6cfd4b67bfd58f24867f765a4445bf2e Mon Sep 17 00:00:00 2001
From: Ramkumar Ramachandra <ramkumar.ramachandra at codasip.com>
Date: Thu, 19 Jun 2025 12:07:55 +0100
Subject: [PATCH 2/2] [HashRecognize] Rewrite arePHIsIntertwined
The test crc8.le.tc16 is a valid CRC algorithm, but isn't recognized as
such due to a buggy arePHIsIntertwined, which is asymmetric in its PHI
node arguments. Another issue is that arePHIsIntertwined is
unnecessarily complicated: the core functionality is to match a BinOp
that's a recurrence in both PHI nodes, ignoring zext and trunc casts.
Hence, rewrite it for clarity, and make it symmetric in both PHI
operands.
crc8.le.tc16 is still not recognized as a valid CRC algorithm, due to an
incorrect check for loop iterations exceeding the bitwidth of the
result: in reality, it should not exceed the bitwidth of LHSAux, but we
leave this fix to a follow-up.
---
llvm/lib/Analysis/HashRecognize.cpp | 65 +++++++------------
.../HashRecognize/cyclic-redundancy-check.ll | 12 ++--
2 files changed, 30 insertions(+), 47 deletions(-)
diff --git a/llvm/lib/Analysis/HashRecognize.cpp b/llvm/lib/Analysis/HashRecognize.cpp
index 1a5dfbe4bed6e..3c877212f777b 100644
--- a/llvm/lib/Analysis/HashRecognize.cpp
+++ b/llvm/lib/Analysis/HashRecognize.cpp
@@ -497,45 +497,29 @@ CRCTable HashRecognize::genSarwateTable(const APInt &GenPoly,
return Table;
}
-/// Checks if \p Reference is reachable from \p Needle on the use-def chain, and
-/// that there are no stray PHI nodes while digging the use-def chain. \p
-/// BOToMatch is a CRC peculiarity: at least one of the Users of Needle needs to
-/// match this OpCode, which is XOR for CRC.
-static bool arePHIsIntertwined(
- const PHINode *Needle, const PHINode *Reference, const Loop &L,
- Instruction::BinaryOps BOToMatch = Instruction::BinaryOpsEnd) {
- // Initialize the worklist with Users of the Needle.
- SmallVector<const Instruction *> Worklist;
- for (const User *U : Needle->users()) {
- if (auto *UI = dyn_cast<Instruction>(U))
- if (L.contains(UI))
- Worklist.push_back(UI);
- }
-
- // BOToMatch is usually XOR for CRC.
- if (BOToMatch != Instruction::BinaryOpsEnd) {
- if (count_if(Worklist, [BOToMatch](const Instruction *I) {
- return I->getOpcode() == BOToMatch;
- }) != 1)
- return false;
- }
-
- while (!Worklist.empty()) {
- const Instruction *I = Worklist.pop_back_val();
-
- // Since Needle is never pushed onto the Worklist, I must either be the
- // Reference PHI node (in which case we're done), or a stray PHI node (in
- // which case we abort).
- if (isa<PHINode>(I))
- return I == Reference;
+/// A small helper for arePHIsIntertwined that is asymmetric in \p L and \p R,
+/// and hence needs to be called twice.
+static bool isXoredWith(const Value *L, const Value *R) {
+ return count_if(L->users(), [L, R](const User *U) {
+ return match(U, m_c_Xor(m_Specific(L),
+ m_CombineOr(m_ZExtOrSelf(m_Specific(R)),
+ m_Trunc(m_Specific(R)))));
+ }) == 1;
+}
- for (const Use &U : I->operands())
- if (auto *UI = dyn_cast<Instruction>(U))
- // Don't push Needle back onto the Worklist.
- if (UI != Needle && L.contains(UI))
- Worklist.push_back(UI);
- }
- return false;
+/// Checks that \p L and \p R are used together in an XOR, which is a recurrence
+/// over both, ignoring zext and trunc. In other words, it checks for the
+/// following pattern:
+///
+/// loop:
+/// %L = phi [_, %entry], [%L.next, %loop]
+/// %R = phi [_, %entry], [%R.next, %loop]
+/// ...
+/// _ = xor ((trunc|zext|self) %L) ((trunc|zext|self) %R)
+///
+/// where at least one cast is self.
+static bool arePHIsIntertwined(const PHINode *L, const PHINode *R) {
+ return isXoredWith(L, R) || isXoredWith(R, L);
}
// Recognizes a multiplication or division by the constant two, using SCEV. By
@@ -586,9 +570,8 @@ HashRecognize::recognizeCRC() const {
if (SimpleRecurrence) {
if (isBigEndianBitShift(SimpleRecurrence.BO, SE) != ByteOrderSwapped)
return "Loop with non-unit bitshifts";
- if (!arePHIsIntertwined(SimpleRecurrence.Phi, ConditionalRecurrence.Phi, L,
- Instruction::BinaryOps::Xor))
- return "Simple recurrence doesn't use conditional recurrence with XOR";
+ if (!arePHIsIntertwined(SimpleRecurrence.Phi, ConditionalRecurrence.Phi))
+ return "Recurrences not intertwined with XOR";
}
// Make sure that the computed value is used in the exit block: this should be
diff --git a/llvm/test/Analysis/HashRecognize/cyclic-redundancy-check.ll b/llvm/test/Analysis/HashRecognize/cyclic-redundancy-check.ll
index 1fa76cb23173b..c42985037d83a 100644
--- a/llvm/test/Analysis/HashRecognize/cyclic-redundancy-check.ll
+++ b/llvm/test/Analysis/HashRecognize/cyclic-redundancy-check.ll
@@ -147,7 +147,7 @@ exit: ; preds = %loop
define i8 @crc8.le.tc16(i16 %msg, i8 %checksum) {
; CHECK-LABEL: 'crc8.le.tc16'
; CHECK-NEXT: Did not find a hash algorithm
-; CHECK-NEXT: Reason: Simple recurrence doesn't use conditional recurrence with XOR
+; CHECK-NEXT: Reason: Loop iterations exceed bitwidth of result
;
entry:
br label %loop
@@ -629,7 +629,7 @@ exit: ; preds = %loop
define i16 @not.crc.wrong.sb.check.const(i8 %msg, i16 %checksum) {
; CHECK-LABEL: 'not.crc.wrong.sb.check.const'
; CHECK-NEXT: Did not find a hash algorithm
-; CHECK-NEXT: Reason: Simple recurrence doesn't use conditional recurrence with XOR
+; CHECK-NEXT: Reason: Bad RHS of significant-bit-check
;
entry:
br label %loop
@@ -868,7 +868,7 @@ exit: ; preds = %loop
define i16 @not.crc.bad.cast(i8 %msg, i16 %checksum) {
; CHECK-LABEL: 'not.crc.bad.cast'
; CHECK-NEXT: Did not find a hash algorithm
-; CHECK-NEXT: Reason: Simple recurrence doesn't use conditional recurrence with XOR
+; CHECK-NEXT: Reason: Expected bottom 8 bits zero (????????00001011)
;
entry:
br label %loop
@@ -895,7 +895,7 @@ exit: ; preds = %loop
define i32 @not.crc.dead.msg.bad.use(i32 %checksum, i32 %msg) {
; CHECK-LABEL: 'not.crc.dead.msg.bad.use'
; CHECK-NEXT: Did not find a hash algorithm
-; CHECK-NEXT: Reason: Simple recurrence doesn't use conditional recurrence with XOR
+; CHECK-NEXT: Reason: Recurrences not intertwined with XOR
;
entry:
br label %loop
@@ -923,7 +923,7 @@ exit: ; preds = %loop
define i16 @not.crc.dead.msg.no.use(i8 %msg, i16 %checksum) {
; CHECK-LABEL: 'not.crc.dead.msg.no.use'
; CHECK-NEXT: Did not find a hash algorithm
-; CHECK-NEXT: Reason: Simple recurrence doesn't use conditional recurrence with XOR
+; CHECK-NEXT: Reason: Recurrences not intertwined with XOR
;
entry:
br label %loop
@@ -952,7 +952,7 @@ exit: ; preds = %loop
define i32 @not.crc.dead.msg.wrong.op(i32 %checksum, i32 %msg) {
; CHECK-LABEL: 'not.crc.dead.msg.wrong.op'
; CHECK-NEXT: Did not find a hash algorithm
-; CHECK-NEXT: Reason: Simple recurrence doesn't use conditional recurrence with XOR
+; CHECK-NEXT: Reason: Recurrences not intertwined with XOR
;
entry:
br label %loop
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