[PATCH] D102385: [GlobalISel][IRTranslator] Use preferred alignment when creating frame indices.

[Amara Emerson via Phabricator via llvm-commits]

aemerson created this revision.
aemerson added reviewers: paquette, arsenm.
aemerson added a project: LLVM.
Herald added subscribers: hiraditya, rovka.
aemerson requested review of this revision.
Herald added a subscriber: wdng.

By using the the preferred alignment when possible, this can allow local stack slot allocation to avoid materializing a base register for stack slots.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D102385

Files:
  llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp
  llvm/test/CodeGen/AArch64/GlobalISel/arm64-irtranslator.ll


Index: llvm/test/CodeGen/AArch64/GlobalISel/arm64-irtranslator.ll
===================================================================
--- llvm/test/CodeGen/AArch64/GlobalISel/arm64-irtranslator.ll
+++ llvm/test/CodeGen/AArch64/GlobalISel/arm64-irtranslator.ll
@@ -35,7 +35,7 @@
 ; CHECK-NEXT:   - { id: 0, name: ptr1, type: default, offset: 0, size: 8, alignment: 8,
 ; CHECK-NEXT:       stack-id: default, callee-saved-register: '', callee-saved-restored: true,
 ; CHECK-NEXT: debug-info-variable: '', debug-info-expression: '', debug-info-location: '' }
-; CHECK-NEXT:   - { id: 1, name: ptr2, type: default, offset: 0, size: 8, alignment: 1,
+; CHECK-NEXT:   - { id: 1, name: ptr2, type: default, offset: 0, size: 8, alignment: 8,
 ; CHECK-NEXT:       stack-id: default, callee-saved-register: '', callee-saved-restored: true,
 ; CHECK-NEXT:       debug-info-variable: '', debug-info-expression: '', debug-info-location: '' }
 ; CHECK-NEXT:   - { id: 2, name: ptr3, type: default, offset: 0, size: 128, alignment: 8,
@@ -54,6 +54,17 @@
   ret void
 }
 
+define void @alloca_increase_alignment()  {
+; The alignment of the frame index should be 8 here since it's the preferred alignment.
+; CHECK: stack:
+; CHECK:   - { id: 0, name: ai, type: default, offset: 0, size: 8, alignment: 8,
+; CHECK:       stack-id: default, callee-saved-register: '', callee-saved-restored: true,
+; CHECK:       debug-info-variable: '', debug-info-expression: '', debug-info-location: '' }
+  %ai = alloca i8*, align 4
+  ret void
+}
+
+
 ; Tests for br.
 ; CHECK-LABEL: name: uncondbr
 ; CHECK: body:
Index: llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp
===================================================================
--- llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp
+++ llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp
@@ -245,8 +245,32 @@
   // Always allocate at least one byte.
   Size = std::max<uint64_t>(Size, 1u);
 
+  const TargetFrameLowering *TFI = MF->getSubtarget().getFrameLowering();
+  const Align StackAlign = TFI->getStackAlign();
+  Align FIAlignment = AI.getAlign();
+  // The following code mimics the alignment optimizations done in
+  // FunctionLoweringInfo for SelectionDAG.
+  Type *Ty = AI.getAllocatedType();
+  Align TyPrefAlign = MF->getDataLayout().getPrefTypeAlign(Ty);
+  // The "specified" alignment is the alignment written on the alloca,
+  // or the preferred alignment of the type if none is specified.
+  Align SpecifiedAlign = AI.getAlign();
+
+  // If the preferred alignment of the type is higher than the specified
+  // alignment of the alloca, promote the alignment, as long as it doesn't
+  // require realigning the stack.
+  //
+  // FIXME: Do we really want to second-guess the IR in isel?
+  Align Alignment = std::max(std::min(TyPrefAlign, StackAlign), SpecifiedAlign);
+
+  // Static allocas can be folded into the initial stack frame
+  // adjustment. For targets that don't realign the stack, don't
+  // do this if there is an extra alignment requirement.
+  if (TFI->isStackRealignable() || (Alignment <= StackAlign))
+    FIAlignment = Alignment;
+
   int &FI = FrameIndices[&AI];
-  FI = MF->getFrameInfo().CreateStackObject(Size, AI.getAlign(), false, &AI);
+  FI = MF->getFrameInfo().CreateStackObject(Size, FIAlignment, false, &AI);
   return FI;
 }
 


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