[CodeGen][BasicBlockSections] Fix wrong alignment directive placement in

basic block section cases

MachineBlockPlacement pass sets an alignment attribute to the loop
header MBB and this attribute will lead to an alignment directive during
emitting asm. In the case of the basic block section, the alignment
directive is put before the section label, and thus the alignment is set
to the predecessor of the loop header, which is not what we expect and
increases the code size (both inserting nop and set section alignment).

Reviewed By: rahmanl

Differential Revision: https://reviews.llvm.org/D137535

diff --git a/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp b/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
index 5863ce0..10c0eea 100644 (file)
--- a/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
@@ -3655,11 +3655,6 @@ void AsmPrinter::emitBasicBlockStart(const MachineBasicBlock &MBB) {
     }
   }
 
-  // Emit an alignment directive for this block, if needed.
-  const Align Alignment = MBB.getAlignment();
-  if (Alignment != Align(1))
-    emitAlignment(Alignment, nullptr, MBB.getMaxBytesForAlignment());
-
   // Switch to a new section if this basic block must begin a section. The
   // entry block is always placed in the function section and is handled
   // separately.
@@ -3670,6 +3665,11 @@ void AsmPrinter::emitBasicBlockStart(const MachineBasicBlock &MBB) {
     CurrentSectionBeginSym = MBB.getSymbol();
   }
 
+  // Emit an alignment directive for this block, if needed.
+  const Align Alignment = MBB.getAlignment();
+  if (Alignment != Align(1))
+    emitAlignment(Alignment, nullptr, MBB.getMaxBytesForAlignment());
+
   // If the block has its address taken, emit any labels that were used to
   // reference the block.  It is possible that there is more than one label
   // here, because multiple LLVM BB's may have been RAUW'd to this block after

diff --git a/llvm/test/CodeGen/X86/align-basic-block-sections.mir b/llvm/test/CodeGen/X86/align-basic-block-sections.mir
new file mode 100644 (file)
index 0000000..055a9da
--- /dev/null
+++ b/llvm/test/CodeGen/X86/align-basic-block-sections.mir
@@ -0,0 +1,136 @@
+# Check if the alignment directive is put on the correct place when the basic block section option is used.
+# RUN: llc -start-after=bbsections-prepare  %s -o - | FileCheck %s -check-prefix=CHECK
+
+# How to generate the input:
+# foo.c
+# int test(int a) {
+#   switch (a) {
+#     default:
+#       return 10;
+#     case 1:
+#       a += 1;
+#     case 2:
+#       a *= -1;
+#     case 8:
+#       break;
+#   }
+#   return a;
+# }
+#
+# clang -O0 -S -emit-llvm test.c
+# llc < test.ll -stop-after=bbsections-prepare -align-all-nofallthru-blocks=8 -basic-block-sections=all
+
+
+--- |
+  define i32 @test(i32 noundef %a) {
+  entry:
+    switch i32 %a, label %return [
+      i32 1, label %sw.bb1
+      i32 2, label %sw.bb1
+      i32 8, label %sw.epilog
+    ]
+
+  sw.bb1:                                           ; preds = %entry, %entry
+    br label %sw.epilog
+
+  sw.epilog:                                        ; preds = %sw.bb1, %entry
+    %a.addr.1 = phi i32 [ %a, %entry ], [ -2, %sw.bb1 ]
+    br label %return
+
+  return:                                           ; preds = %sw.epilog, %entry
+    %retval.0 = phi i32 [ %a.addr.1, %sw.epilog ], [ 10, %entry ]
+    ret i32 %retval.0
+  }
+
+
+...
+---
+name:            test
+alignment:       16
+exposesReturnsTwice: false
+legalized:       false
+regBankSelected: false
+selected:        false
+failedISel:      false
+tracksRegLiveness: true
+hasWinCFI:       false
+callsEHReturn:   false
+callsUnwindInit: false
+hasEHCatchret:   false
+hasEHScopes:     false
+hasEHFunclets:   false
+failsVerification: false
+tracksDebugUserValues: true
+registers:       []
+liveins:
+  - { reg: '$edi', virtual-reg: '' }
+frameInfo:
+  isFrameAddressTaken: false
+  isReturnAddressTaken: false
+  hasStackMap:     false
+  hasPatchPoint:   false
+  stackSize:       0
+  offsetAdjustment: 0
+  maxAlignment:    1
+  adjustsStack:    false
+  hasCalls:        false
+  stackProtector:  ''
+  functionContext: ''
+  maxCallFrameSize: 0
+  cvBytesOfCalleeSavedRegisters: 0
+  hasOpaqueSPAdjustment: false
+  hasVAStart:      false
+  hasMustTailInVarArgFunc: false
+  hasTailCall:     false
+  localFrameSize:  0
+  savePoint:       ''
+  restorePoint:    ''
+fixedStack:      []
+stack:           []
+callSites:       []
+debugValueSubstitutions: []
+constants:       []
+machineFunctionInfo: {}
+body:             |
+  bb.0.entry:
+    successors: %bb.1(0x40000000), %bb.2(0x40000000)
+    liveins: $edi
+  
+    renamable $edi = KILL $edi, implicit-def $rdi
+    renamable $eax = LEA64_32r renamable $rdi, 1, $noreg, -1, $noreg
+    CMP32ri8 killed renamable $eax, 2, implicit-def $eflags
+    JCC_1 %bb.2, 3, implicit $eflags
+    JMP_1 %bb.1
+  
+  bb.1.sw.bb1 (bbsections 1):
+    successors: %bb.3(0x80000000)
+  
+    renamable $edi = MOV32ri -2, implicit-def $rdi
+    JMP_1 %bb.3
+  
+  bb.2.entry (align 256, bbsections 2):
+    successors: %bb.3(0x40000000), %bb.4(0x40000000)
+    liveins: $rdi
+  
+    renamable $eax = MOV32ri 10
+    CMP32ri8 renamable $edi, 8, implicit-def $eflags
+    JCC_1 %bb.4, 5, implicit $eflags
+    JMP_1 %bb.3
+  
+  bb.3.sw.epilog (bbsections 3):
+    successors: %bb.4(0x80000000)
+    liveins: $rdi
+  
+    $eax = MOV32rr $edi, implicit killed $rdi
+    JMP_1 %bb.4
+  
+  bb.4.return (bbsections 4):
+    liveins: $eax
+  
+    RET64 $eax
+
+...
+
+# CHECK: .section      .text.test,"ax",@progbits,unique,2
+# CHECK-NEXT: .p2align 8, 0x90
+# CHECK-NEXT: test.__part.2: # %entry