[AArch64] Disable loop alignment for Windows targets (#67894)

This should fix #66912. When emitting SEH unwind info, we need to be
able to calculate the exact length of functions before alignments are
fixed. Until that limitation is overcome, just disable all loop
alignment on Windows targets.

(cherry picked from commit 6ae36c012728a274a78a771e4506681732f85a6d)

diff --git a/llvm/lib/MC/MCWin64EH.cpp b/llvm/lib/MC/MCWin64EH.cpp
index a2d61da..bb3492b 100644 (file)
--- a/llvm/lib/MC/MCWin64EH.cpp
+++ b/llvm/lib/MC/MCWin64EH.cpp
@@ -1402,6 +1402,9 @@ static void ARM64EmitUnwindInfo(MCStreamer &streamer, WinEH::FrameInfo *info,
     // here, but we'd have to emit the pdata, the xdata header, and the
     // epilogue scopes later, since they depend on whether the we need to
     // split the unwind data.
+    //
+    // If this is fixed, remove code in AArch64ISelLowering.cpp that
+    // disables loop alignment on Windows.
     RawFuncLength = GetAbsDifference(streamer, info->FuncletOrFuncEnd,
                                      info->Begin);
   }

diff --git a/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp b/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
index a1753a4..6e721b9 100644 (file)
--- a/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
+++ b/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
@@ -1033,7 +1033,12 @@ AArch64TargetLowering::AArch64TargetLowering(const TargetMachine &TM,
   // Set required alignment.
   setMinFunctionAlignment(Align(4));
   // Set preferred alignments.
-  setPrefLoopAlignment(STI.getPrefLoopAlignment());
+
+  // Don't align loops on Windows. The SEH unwind info generation needs to
+  // know the exact length of functions before the alignments have been
+  // expanded.
+  if (!Subtarget->isTargetWindows())
+    setPrefLoopAlignment(STI.getPrefLoopAlignment());
   setMaxBytesForAlignment(STI.getMaxBytesForLoopAlignment());
   setPrefFunctionAlignment(STI.getPrefFunctionAlignment());
 

diff --git a/llvm/test/CodeGen/AArch64/sched-loop-align.ll b/llvm/test/CodeGen/AArch64/sched-loop-align.ll
new file mode 100644 (file)
index 0000000..5b8e42c
--- /dev/null
+++ b/llvm/test/CodeGen/AArch64/sched-loop-align.ll
@@ -0,0 +1,21 @@
+; RUN: llc < %s -mtriple=aarch64-windows | FileCheck %s --check-prefix=WINDOWS
+; RUN: llc < %s -mtriple=aarch64-linux | FileCheck %s --check-prefix=LINUX
+
+define dso_local void @b() #0 {
+entry:
+  br label %for.cond
+
+for.cond:
+  tail call void @a()
+  br label %for.cond
+}
+
+declare dso_local void @a(...)
+
+attributes #0 = { noreturn nounwind uwtable "tune-cpu"="cortex-a53" }
+
+; LINUX-LABEL: b:
+; LINUX: .p2align 4
+
+; WINDOWS-LABEL: b:
+; WINDOWS-NOT: .p2align