[MachineLoopInfo] Fix getLoopID to handle multi latches. (#106195)

This patch also fixed `CodegenPrepare` to preserve loop metadata when
merging blocks.

This fixes issue #102632

Author: FreddyLeaf

llvm/lib/CodeGen/CodeGenPrepare.cpp

@@ -1183,6 +1183,12 @@ void CodeGenPrepare::eliminateMostlyEmptyBlock(BasicBlock *BB) {
     }
   }
 
+  // Preserve loop Metadata.
+  if (BI->hasMetadata(LLVMContext::MD_loop)) {
+    for (auto *Pred : predecessors(BB))
+      Pred->getTerminator()->copyMetadata(*BI, LLVMContext::MD_loop);
+  }
+
   // The PHIs are now updated, change everything that refers to BB to use
   // DestBB and remove BB.
   BB->replaceAllUsesWith(DestBB);

llvm/lib/CodeGen/MachineLoopInfo.cpp

@@ -212,7 +212,7 @@ MDNode *MachineLoop::getLoopID() const {
           }
         }
         if (!MD)
-          return nullptr;
+          continue;
         if (!LoopID)
           LoopID = MD;
         else if (MD != LoopID)

llvm/test/CodeGen/X86/code-align-loops.ll

@@ -96,8 +96,8 @@ for.body5:                                        ; preds = %for.body, %for.body
   br i1 %exitcond16.not, label %for.cond.cleanup4, label %for.body5, !llvm.loop !2
 }
 
-; test3 is to check if .p2align can be correctly set on loops with multi latches.
-; The test IR is generated from below simple C file:
+; test3 and test4 is to check if .p2align can be correctly set on loops with
+; multi latches. The IR is generated from below simple C file:
 ; $ clang -O0 -S -emit-llvm loop.c
 ; $ cat loop.c
 ; int test3() {
@@ -111,7 +111,7 @@ for.body5:                                        ; preds = %for.body, %for.body
 ;     }
 ; }
 ; CHECK-LABEL: test3_multilatch:
-; ALIGN: .p2align 4, 0x90
+; ALIGN: .p2align 6, 0x90
 ; ALIGN-NEXT: .LBB2_1: # %while.cond
 define dso_local i32 @test3_multilatch() #0 {
 entry:
@@ -146,6 +146,37 @@ while.end:                                        ; preds = %while.cond
   ret i32 %3
 }
 
+; CHECK-LABEL: test4_multilatch:
+; ALIGN: .p2align 6, 0x90
+; ALIGN-NEXT: .LBB3_4: # %bb4
+define void @test4_multilatch(i32 %a, i32 %b, i32 %c, i32 %d) nounwind {
+entry:
+  br label %bb1
+
+bb1:                               ; preds = %bb2, %bb4, %entry
+  call void @bar()
+  %cmp3 = icmp sgt i32 %c, 10
+  br i1 %cmp3, label %bb3, label %bb4
+
+bb2:                                ; preds = %bb3
+  call void @bar()
+  %cmp1 = icmp sgt i32 %a, 11
+  br i1 %cmp1, label %bb1, label %exit, !llvm.loop !0
+
+bb3:                                ; preds = %bb1
+  call void @bar()
+  %cmp2 = icmp sgt i32 %b, 12
+  br i1 %cmp2, label %bb2, label %exit
+
+bb4:                                ; preds = %bb1
+  call void @bar()
+  %cmp4 = icmp sgt i32 %d, 14
+  br i1 %cmp4, label %bb1, label %exit
+
+exit:                               ; preds = %bb2, %bb3, %bb4
+  ret void
+}
+
 declare void @bar()
 declare void @var()