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[CodeGen][MachineLoop] Fix getLoopID #137820

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Merged
merged 4 commits into from
May 24, 2025
Merged

[CodeGen][MachineLoop] Fix getLoopID #137820

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c0d1161
[CodeGen][MachineLoop] Fix getLoopID
kortbeek-snps Apr 29, 2025
a4bb987
[MachineBlockPlacement][X86] Add test for loops with single latch tha…
kortbeek-snps Apr 29, 2025
a31fac8
[MachineBlockPlacement][X86] Address test comments
kortbeek-snps May 23, 2025
7f5a5f4
[CodeGen][MachineLoop] Address code comment
kortbeek-snps May 23, 2025
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60 changes: 22 additions & 38 deletions llvm/lib/CodeGen/MachineLoopInfo.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -181,48 +181,32 @@ MachineLoopInfo::findLoopPreheader(MachineLoop *L, bool SpeculativePreheader,

MDNode *MachineLoop::getLoopID() const {
MDNode *LoopID = nullptr;
if (const auto *MBB = findLoopControlBlock()) {
// If there is a single latch block, then the metadata
// node is attached to its terminating instruction.

// Go through the latch blocks and check the terminator for the metadata
SmallVector<MachineBasicBlock *, 4> LatchesBlocks;
getLoopLatches(LatchesBlocks);
for (const auto *MBB : LatchesBlocks) {
const auto *BB = MBB->getBasicBlock();
if (!BB)
return nullptr;
if (const auto *TI = BB->getTerminator())
LoopID = TI->getMetadata(LLVMContext::MD_loop);
} else if (const auto *MBB = getHeader()) {
// There seem to be multiple latch blocks, so we have to
// visit all predecessors of the loop header and check
// their terminating instructions for the metadata.
if (const auto *Header = MBB->getBasicBlock()) {
// Walk over all blocks in the loop.
for (const auto *MBB : this->blocks()) {
const auto *BB = MBB->getBasicBlock();
if (!BB)
return nullptr;
const auto *TI = BB->getTerminator();
if (!TI)
return nullptr;
MDNode *MD = nullptr;
// Check if this terminating instruction jumps to the loop header.
for (const auto *Succ : successors(TI)) {
if (Succ == Header) {
// This is a jump to the header - gather the metadata from it.
MD = TI->getMetadata(LLVMContext::MD_loop);
break;
}
}
if (!MD)
continue;
if (!LoopID)
LoopID = MD;
else if (MD != LoopID)
return nullptr;
}
}
const auto *TI = BB->getTerminator();
if (!TI)
return nullptr;

MDNode *MD = TI->getMetadata(LLVMContext::MD_loop);
if (!MD)
return nullptr;

if (!LoopID)
LoopID = MD;
else if (MD != LoopID)
return nullptr;
}
if (LoopID &&
(LoopID->getNumOperands() == 0 || LoopID->getOperand(0) != LoopID))
LoopID = nullptr;

if (!LoopID || LoopID->getNumOperands() == 0 ||
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Is the getNumOperands() == 0 check really necessary? I would hope the verifier rejects cases with missing operands (but you are just moving existing code)

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I was indeed just moving code.

However, I don't find any checks for this in the verifier, but I'm not too familiar with it. If I just experimentally remove the LoopID operands and run opt -passes=verify code-align-loops.ll -disable-output (or llc), I get an assert during the LLVMIR parsing:
llvm/lib/IR/DebugInfo.cpp:536: MDNode *stripDebugLocFromLoopID(MDNode *): Assertion !N->operands().empty() && "Missing self reference?"' failed.

I do agree that it feels a bit out of place to check the validity of the metadata here, but since LoopInfo.cpp::Loop::getLoopID() does the same, It might not be too strange?

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Unrelated to this pr, the IR verifier should be verifying these. The MIR verifier should also, but that is more awkward

LoopID->getOperand(0) != LoopID)
return nullptr;

return LoopID;
}

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42 changes: 42 additions & 0 deletions llvm/test/CodeGen/X86/code-align-loops.ll
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Expand Up @@ -177,6 +177,48 @@ exit: ; preds = %bb2, %bb3, %bb4
ret void
}

; test5 is to check if .p2align can be correctly set on loops with a single
; latch that's not the exiting block.
; The test IR is generated from below simple C file:
; $ clang -O0 -S -emit-llvm loop.c
; $ cat loop.c
; int test5(int n) {
; int i = 0;
; [[clang::code_align(64)]]
; while (i < n) {
; i++;
; }
; }
; CHECK-LABEL: test5:
; ALIGN: .p2align 6
; ALIGN-NEXT: .LBB4_1: # %while.cond
define i32 @test5(i32 %n) #0 {
entry:
%retval = alloca i32, align 4
%n.addr = alloca i32, align 4
%i = alloca i32, align 4
store i32 %n, ptr %n.addr, align 4
store i32 0, ptr %i, align 4
br label %while.cond

while.cond: ; preds = %while.body, %entry
%i.val = load i32, ptr %i, align 4
%n.val = load i32, ptr %n.addr, align 4
%cmp = icmp slt i32 %i.val, %n.val
br i1 %cmp, label %while.body, label %while.end

while.body: ; preds = %while.cond
%tmp = load i32, ptr %i, align 4
%inc = add nsw i32 %tmp, 1
store i32 %inc, ptr %i, align 4
br label %while.cond, !llvm.loop !0

while.end: ; preds = %while.cond
%val = load i32, ptr %retval, align 4
ret i32 %val
}


declare void @bar()
declare void @var()

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