[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()