[CodeGen][MachineLoop] Fix getLoopID (#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
(#71026 (comment))
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
#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 !

Author: kortbeek-snps

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;
 }
 

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 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()