[llvm] [CodeGen][MachineLoop] Fix getLoopID (PR #137820)
Vito Kortbeek via llvm-commits
llvm-commits at lists.llvm.org
Tue Apr 29 07:37:26 PDT 2025
https://github.com/kortbeek-snps created https://github.com/llvm/llvm-project/pull/137820
Mirror the `getLoopID()` implementation of `LoopInfo` in `MachineLoopInfo`.
`getLoopID` used `findLoopControlBlock` to detect the special case where there is a single latch. However, `findLoopControlBlock` returns the exiting block if the latch is not an exiting block. The middle end places the `LoopID` metadata on the
latch regardless of if it's an exiting block or not.
I raised this issue in the PR that introduced the `getLoopID()` helper (https://github.com/llvm/llvm-project/pull/71026#discussion_r2000595214) and @FreddyLeaf confirmed this is a bug and asked me to help implement a refinement.
I've mirrored the implementation of `LoopInfo` instead of simply changing `findLoopControlBlock()` to `findLoopControlBlock()` to keep the two implementations consistent. The only difference between the two is that `MachineLoopInfo::getLoopID` initially starts out with a `MachineBacisBlock` and attempts to retrieve the `BasicBlock` (if it wasn't for this difference, I would have moved it to `genericLoopInfo`).
I've also updated the test associated with https://github.com/llvm/llvm-project/pull/71026 (`test5`) that check the alignment for a loop with a single latch that's not the exit. This test will fail for the current implementation. I'm not sure if we want to include this test upstream (it might look out of place after we remove the 'single-latch-specialization' from `getLoopID()`).
Let me know if you have any comments, @FreddyLeaf !
>From c0d11615319de3e5bc12de7b5cc5e5628888ad10 Mon Sep 17 00:00:00 2001
From: Vito Kortbeek <kortbeek at synopsys.com>
Date: Tue, 29 Apr 2025 16:13:20 +0200
Subject: [PATCH 1/2] [CodeGen][MachineLoop] Fix getLoopID
Mirror the getLoopID implementation of LoopInfo in MachineLoopInfo.
getLoopID used findLoopControlBlock to detect the special case where there is a
single latch. However, findLoopControlBlock returns the exiting block if the
latch is not an exiting block. The middle end places the LoopID metadata on the
latch regardless of if it's an exiting block or not.
---
llvm/lib/CodeGen/MachineLoopInfo.cpp | 60 ++++++++++------------------
1 file changed, 22 insertions(+), 38 deletions(-)
diff --git a/llvm/lib/CodeGen/MachineLoopInfo.cpp b/llvm/lib/CodeGen/MachineLoopInfo.cpp
index d6906bacde0e2..4fc7fe4d52417 100644
--- a/llvm/lib/CodeGen/MachineLoopInfo.cpp
+++ b/llvm/lib/CodeGen/MachineLoopInfo.cpp
@@ -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 ||
+ LoopID->getOperand(0) != LoopID)
+ return nullptr;
+
return LoopID;
}
>From a4bb98714e904cd71164403b11d1962f44fa1851 Mon Sep 17 00:00:00 2001
From: Vito Kortbeek <kortbeek at synopsys.com>
Date: Tue, 29 Apr 2025 16:20:15 +0200
Subject: [PATCH 2/2] [MachineBlockPlacement][X86] Add test for loops with
single latch that's not exiting
---
llvm/test/CodeGen/X86/code-align-loops.ll | 42 +++++++++++++++++++++++
1 file changed, 42 insertions(+)
diff --git a/llvm/test/CodeGen/X86/code-align-loops.ll b/llvm/test/CodeGen/X86/code-align-loops.ll
index 68ae49792ed18..f3f36f575a37c 100644
--- a/llvm/test/CodeGen/X86/code-align-loops.ll
+++ b/llvm/test/CodeGen/X86/code-align-loops.ll
@@ -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 dso_local 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
+ %0 = load i32, ptr %i, align 4
+ %1 = load i32, ptr %n.addr, align 4
+ %cmp = icmp slt i32 %0, %1
+ br i1 %cmp, label %while.body, label %while.end
+
+while.body: ; preds = %while.cond
+ %2 = load i32, ptr %i, align 4
+ %inc = add nsw i32 %2, 1
+ store i32 %inc, ptr %i, align 4
+ br label %while.cond, !llvm.loop !0
+
+while.end: ; preds = %while.cond
+ %3 = load i32, ptr %retval, align 4
+ ret i32 %3
+}
+
+
declare void @bar()
declare void @var()
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