CodeGen: Emit .prefalign directives based on the prefalign attribute.

lld/test/ELF/lto/linker-script-symbols-ipo.ll

@@ -18,7 +18,7 @@
 ; NOIPO:      <foo>:
 ; NOIPO-NEXT:   movl $2, %eax
 ; NOIPO:      <_start>:
-; NOIPO-NEXT:   jmp 0x201160 <foo>
+; NOIPO-NEXT:   jmp 0x201158 <foo>
 
 target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"

llvm/include/llvm/CodeGen/MachineFunction.h

@@ -818,6 +818,8 @@ class LLVM_ABI MachineFunction {
       Alignment = A;
   }
 
+  Align getPreferredAlignment() const;
+
   /// exposesReturnsTwice - Returns true if the function calls setjmp or
   /// any other similar functions with attribute "returns twice" without
   /// having the attribute itself.

llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp

@@ -977,8 +977,20 @@ void AsmPrinter::emitFunctionHeader() {
     emitVisibility(CurrentFnSym, F.getVisibility());
 
   emitLinkage(&F, CurrentFnSym);
-  if (MAI->hasFunctionAlignment())
-    emitAlignment(MF->getAlignment(), &F);
+  if (MAI->hasFunctionAlignment()) {
+    // The preferred alignment directive will not have the intended effect
+    // unless function sections are enabled.
+    if (MAI->useIntegratedAssembler() && MAI->hasPreferredAlignment() &&
+        TM.getFunctionSections()) {
+      Align Alignment = MF->getAlignment();
+      Align PrefAlignment = MF->getPreferredAlignment();
+      emitAlignment(Alignment, &F);
+      if (Alignment != PrefAlignment)
+        OutStreamer->emitPrefAlign(PrefAlignment);
+    } else {
+      emitAlignment(MF->getPreferredAlignment(), &F);
+    }
+  }
 
   if (MAI->hasDotTypeDotSizeDirective())
     OutStreamer->emitSymbolAttribute(CurrentFnSym, MCSA_ELF_TypeFunction);

llvm/lib/CodeGen/AsmPrinter/WinException.cpp

@@ -204,8 +204,8 @@ void WinException::beginFunclet(const MachineBasicBlock &MBB,
 
     // We want our funclet's entry point to be aligned such that no nops will be
     // present after the label.
-    Asm->emitAlignment(std::max(Asm->MF->getAlignment(), MBB.getAlignment()),
-                       &F);
+    Asm->emitAlignment(
+        std::max(Asm->MF->getPreferredAlignment(), MBB.getAlignment()), &F);
 
     // Now that we've emitted the alignment directive, point at our funclet.
     Asm->OutStreamer->emitLabel(Sym);

llvm/lib/CodeGen/MachineFunction.cpp

@@ -211,11 +211,6 @@ void MachineFunction::init() {
   ConstantPool = new (Allocator) MachineConstantPool(getDataLayout());
   Alignment = STI.getTargetLowering()->getMinFunctionAlignment();
 
-  // FIXME: Shouldn't use pref alignment if explicit alignment is set on F.
-  if (!F.hasOptSize())
-    Alignment = std::max(Alignment,
-                         STI.getTargetLowering()->getPrefFunctionAlignment());
-
   // -fsanitize=function and -fsanitize=kcfi instrument indirect function calls
   // to load a type hash before the function label. Ensure functions are aligned
   // by a least 4 to avoid unaligned access, which is especially important for
@@ -330,6 +325,18 @@ bool MachineFunction::shouldSplitStack() const {
   return getFunction().hasFnAttribute("split-stack");
 }
 
+Align MachineFunction::getPreferredAlignment() const {
+  Align PrefAlignment = Align(1);
+
+  if (!F.hasOptSize())
+    PrefAlignment = STI.getTargetLowering()->getPrefFunctionAlignment();
+
+  if (MaybeAlign A = F.getPreferredAlignment())
+    PrefAlignment = *A;
+
+  return std::max(PrefAlignment, getAlignment());
+}
+
 [[nodiscard]] unsigned
 MachineFunction::addFrameInst(const MCCFIInstruction &Inst) {
   FrameInstructions.push_back(Inst);

llvm/lib/Target/X86/X86AsmPrinter.cpp

@@ -155,7 +155,7 @@ void X86AsmPrinter::EmitKCFITypePadding(const MachineFunction &MF,
   if (HasType)
     PrefixBytes += 5;
 
-  emitNops(offsetToAlignment(PrefixBytes, MF.getAlignment()));
+  emitNops(offsetToAlignment(PrefixBytes, MF.getPreferredAlignment()));
 }
 
 /// emitKCFITypeId - Emit the KCFI type information in architecture specific

llvm/test/CodeGen/PowerPC/alloca-crspill.ll

@@ -26,7 +26,7 @@ entry:
 declare signext i32 @do_something(ptr)
 
 ; CHECK: name:            test
-; CHECK: alignment:       16
+; CHECK: alignment:       4
 ; CHECK: liveins:
 ; CHECK64:   - { reg: '$x3', virtual-reg: '' }
 ; CHECK32:   - { reg: '$r3', virtual-reg: '' }

llvm/test/CodeGen/X86/prefalign.ll

@@ -0,0 +1,27 @@
+; RUN: llc < %s | FileCheck --check-prefixes=CHECK,NOFS %s
+; RUN: llc -function-sections < %s | FileCheck --check-prefixes=CHECK,FS %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+; CHECK: .globl f1
+; NOFS-NEXT: .p2align 4
+; FS-NEXT: .prefalign 16
+define void @f1() {
+  ret void
+}
+
+; CHECK: .globl f2
+; CHECK-NOT: .prefalign
+; CHECK-NOT: .p2align
+define void @f2() prefalign 1 {
+  ret void
+}
+
+; CHECK: .globl f3
+; NOFS-NEXT: .p2align 2
+; FS-NEXT: .p2align 1
+; FS-NEXT: .prefalign 4
+define void @f3() align 2 prefalign 4 {
+  ret void
+}

llvm/test/Transforms/SampleProfile/pseudo-probe-emit.ll

@@ -107,7 +107,7 @@ entry:
 ; CHECK-SEC:       [Nr] Name               Type     {{.*}} ES Flg Lk Inf Al
 ; CHECK-SEC:       [ 3] .text.foo          PROGBITS {{.*}} 00  AX  0   0 16
 ; CHECK-SEC:       [ 5] .text.foo2         PROGBITS {{.*}} 00  AX  0   0 16
-; CHECK-SEC:       [ 8] .text.foo3         PROGBITS {{.*}} 00  AXG 0   0 16
+; CHECK-SEC:       [ 8] .text.foo3         PROGBITS {{.*}} 00  AXG 0   0  1
 ; CHECK-SEC-COUNT-3:    .pseudo_probe_desc PROGBITS
 ; CHECK-SEC:            .pseudo_probe      PROGBITS {{.*}} 00   L  3   0  1
 ; CHECK-SEC-NEXT:       .pseudo_probe      PROGBITS {{.*}} 00   L  5   0  1
@@ -135,7 +135,7 @@ entry:
 ; CHECK-SEC2:      [Nr] Name               Type     {{.*}} ES Flg Lk Inf Al
 ; CHECK-SEC2:      [ 3] .text              PROGBITS {{.*}} 00  AX  0   0 16
 ; CHECK-SEC2:      [ 5] .text              PROGBITS {{.*}} 00  AX  0   0 16
-; CHECK-SEC2:      [ 8] .text              PROGBITS {{.*}} 00  AXG 0   0 16
+; CHECK-SEC2:      [ 8] .text              PROGBITS {{.*}} 00  AXG 0   0  1
 ; CHECK-SEC2-COUNT-3:   .pseudo_probe_desc PROGBITS
 ; CHECK-SEC2:           .pseudo_probe      PROGBITS {{.*}} 00   L  3   0  1
 ; CHECK-SEC2-NEXT:      .pseudo_probe      PROGBITS {{.*}} 00   L  5   0  1