[PATCH] Fix AArch64 prologue for empty frame with dynamic allocas

[Evgeniy Stepanov]

Hi kristof.beyls,

Fixes PR23804: assertion failure in emitPrologue in the case of a function with an empty frame and a dynamic alloca that needs stack realignment.
This is a typical case for AddressSanitizer.

REPOSITORY
  rL LLVM

http://reviews.llvm.org/D10826

Files:
  lib/Target/AArch64/AArch64FrameLowering.cpp
  test/CodeGen/AArch64/aarch64-dynamic-stack-layout-pr23804.ll

Index: lib/Target/AArch64/AArch64FrameLowering.cpp
===================================================================
--- lib/Target/AArch64/AArch64FrameLowering.cpp
+++ lib/Target/AArch64/AArch64FrameLowering.cpp
@@ -351,7 +351,7 @@
   const unsigned Alignment = MFI->getMaxAlignment();
   const bool NeedsRealignment = (Alignment > 16);
   unsigned scratchSPReg = AArch64::SP;
-  if (NeedsRealignment) {
+  if (NumBytes && NeedsRealignment) {
     // Use the first callee-saved register as a scratch register
     assert(MF.getRegInfo().isPhysRegUsed(AArch64::X9) &&
            "No scratch register to align SP!");
@@ -366,9 +366,6 @@
     emitFrameOffset(MBB, MBBI, DL, scratchSPReg, AArch64::SP, -NumBytes, TII,
                     MachineInstr::FrameSetup);
 
-  assert(!(NeedsRealignment && NumBytes==0) &&
-         "NumBytes should never be 0 when realignment is needed");
-
   if (NumBytes && NeedsRealignment) {
     const unsigned NrBitsToZero = countTrailingZeros(Alignment);
     assert(NrBitsToZero > 1);
Index: test/CodeGen/AArch64/aarch64-dynamic-stack-layout-pr23804.ll
===================================================================
--- test/CodeGen/AArch64/aarch64-dynamic-stack-layout-pr23804.ll
+++ test/CodeGen/AArch64/aarch64-dynamic-stack-layout-pr23804.ll
@@ -0,0 +1,26 @@
+; RUN: llc -verify-machineinstrs -mtriple=aarch64-none-linux-gnu < %s | FileCheck %s
+
+
+
+define void @realign_conditional(i1 %b) {
+entry:
+  br i1 %b, label %bb0, label %bb1
+
+bb0:
+  %MyAlloca = alloca i8, i64 64, align 32
+  br label %bb1
+
+bb1:
+  ret void
+}
+
+; CHECK-LABEL: realign_conditional
+; No realignment in the prologue.
+; CHECK-NOT:  and
+; CHECK-NOT:  0xffffffffffffffe0
+; CHECK:  tbz  {{.*}}[[LABEL:.*]]
+; Stack is realigned in a non-entry BB.
+; CHECK:  sub  x8, sp, #64
+; CHECK:  and  sp, x8, #0xffffffffffffffe0
+; CHECK:  [[LABEL]]:
+; CHECK:  ret

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