BranchRelaxation: Account for function alignment

[hfinkel]

You need to update the comment, but otherwise this LGTM.

lib/CodeGen/BranchRelaxation.cpp

51

Please update the comment to say that it takes the MBB of the following block.

[arsenm]

r283462

lib/CodeGen/BranchRelaxation.cpp

struct BasicBlockInfo {
     /// The size does not include any alignment padding whether from the
     /// beginning of the block, or from an aligned jump table at the end.
     unsigned Size;
 
     BasicBlockInfo() : Offset(0), Size(0) {}
 
     /// Compute the offset immediately following this block.  If LogAlign is
     /// specified, return the offset the successor block will get if it has
     /// this alignment.
-    unsigned postOffset(unsigned LogAlign = 0) const {
+    unsigned postOffset(const MachineBasicBlock &MBB) const {
       unsigned PO = Offset + Size;
-      unsigned Align = 1 << LogAlign;
-      return (PO + Align - 1) / Align * Align;
+      unsigned Align = MBB.getAlignment();
+      if (Align == 0)
+        return PO;
+
+      unsigned AlignAmt = 1 << Align;
+      unsigned ParentAlign = MBB.getParent()->getAlignment();
+      if (Align <= ParentAlign)
+        return PO + OffsetToAlignment(PO, AlignAmt);
+
+      // The alignment of this MBB is larger than the function's alignment, so we
+      // can't tell whether or not it will insert nops. Assume that it will.
+      return PO + AlignAmt + OffsetToAlignment(PO, AlignAmt);
     }
   };
 
   SmallVector<BasicBlockInfo, 16> BlockInfo;
 
   MachineFunction *MF;
   const TargetInstrInfo *TII;
 

 /// verify - check BBOffsets, BBSizes, alignment of islands
 void BranchRelaxation::verify() {
 #ifndef NDEBUG
   unsigned PrevNum = MF->begin()->getNumber();
   for (MachineBasicBlock &MBB : *MF) {
     unsigned Align = MBB.getAlignment();
     unsigned Num = MBB.getNumber();
     assert(BlockInfo[Num].Offset % (1u << Align) == 0);
-    assert(!Num || BlockInfo[PrevNum].postOffset() <= BlockInfo[Num].Offset);
+    assert(!Num || BlockInfo[PrevNum].postOffset(MBB) <= BlockInfo[Num].Offset);
     PrevNum = Num;
   }
 #endif
 }
 
 /// print block size and offset information - debugging
 void BranchRelaxation::dumpBBs() {
   for (auto &MBB : *MF) {

 void BranchRelaxation::adjustBlockOffsets(MachineBasicBlock &Start) {
   unsigned PrevNum = Start.getNumber();
   for (auto &MBB : make_range(MachineFunction::iterator(Start), MF->end())) {
     unsigned Num = MBB.getNumber();
     if (!Num) // block zero is never changed from offset zero.
       continue;
     // Get the offset and known bits at the end of the layout predecessor.
     // Include the alignment of the current block.
-    unsigned LogAlign = MBB.getAlignment();
-    BlockInfo[Num].Offset = BlockInfo[PrevNum].postOffset(LogAlign);
+    BlockInfo[Num].Offset = BlockInfo[PrevNum].postOffset(MBB);
+
     PrevNum = Num;
   }
 }
 
 /// Split the basic block containing MI into two blocks, which are joined by
 /// an unconditional branch.  Update data structures and renumber blocks to
 /// account for this change and returns the newly created block.
 /// NOTE: Successor list of the original BB is out of date after this function,

test/CodeGen/AArch64/branch-relax-alignment.ll

+; RUN: llc -mtriple=aarch64-apple-darwin -aarch64-bcc-offset-bits=4 -align-all-nofallthru-blocks=4 < %s | FileCheck %s
+
+; Long branch is assumed because the block has a higher alignment
+; requirement than the function.
+
+; CHECK-LABEL: invert_bcc_block_align_higher_func:
+; CHECK: b.eq [[JUMP_BB1:LBB[0-9]+_[0-9]+]]
+; CHECK-NEXT: b [[JUMP_BB2:LBB[0-9]+_[0-9]+]]
+
+; CHECK: [[JUMP_BB1]]:
+; CHECK: ret
+; CHECK: .p2align 4
+
+; CHECK: [[JUMP_BB2]]:
+; CHECK: ret
+define i32 @invert_bcc_block_align_higher_func(i32 %x, i32 %y) align 4 #0 {
+  %1 = icmp eq i32 %x, %y
+  br i1 %1, label %bb1, label %bb2
+
+bb2:
+  store volatile i32 9, i32* undef
+  ret i32 1
+
+bb1:
+  store volatile i32 42, i32* undef
+  ret i32 0
+}
+
+attributes #0 = { nounwind }
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