Fix interaction between stack alignment and inline-asm stack clash protection

[serge-sans-paille]

Summary

As reported in https://github.com/rust-lang/rust/issues/70143 alignment is not taken into account when doing the probing. Fix that by adjusting the first probe if the stack align is small, or by extending the dynamic probing if the alignment is large.

[lkail]

llvm/lib/Target/X86/X86FrameLowering.cpp

672

When Align > StackProbeSize, which should be rare, is there any chance that the first probe is performed on the old frame?

[serge-sans-paille]

llvm/lib/Target/X86/X86FrameLowering.cpp

672

That's a concern I have. In that particular code that's ok, because we check Align <= StackProbeSize and we just adjust the first probe. In the other case what we're basically doing is

rsp += rem(rsp, align)
rsp -= align
rsp += StackProbeSize
*rsp = 0

if rsp is already aligned, we end-up doing align/StackProbeSize useless probing

[cuviper]

Can you add tests that show what this is intended to look like for alignments greater than the probe step size?

Right now AFAICS you're just adding the alignment, which doesn't make sense to me. In the Rust code I examined, alignment was achieved by ANDing an alignment mask, which essentially rounds down to the next aligned address. That's a dynamically-sized address range that needs to be part of the probing.

[serge-sans-paille]

Added test case for large stack align

[cuviper]

llvm/test/CodeGen/X86/stack-clash-small-large-align.ll

42 ↗

(On Diff #282221)

There's an immediate race after andq $-65536, %rsp -- if we get a signal here, the stack pointer could be in a potentially bad place and start clobbering stuff.

Then adding the full alignment puts us into arbitrary stack memory, or even could go past the top of the stack. If we start writing probes from there, who knows what memory we're clobbering. (akin to @lkail's concern)

Consider if the stack is almost aligned to begin with, something like 0x1230010. The and will only adjust a small distance to 0x1230000, and then the add makes it 0x1240000. The first probe will be a page below that, 0x123E000, but that's still way out of our frame, not memory we should be writing.

Actually, that plays just as badly if the stack is perfectly aligned to begin with.

The other extreme was the original concern, perhaps with an incoming stack like 0x123FFE0. Then it will again and to 0x1230000, add to 0x1240000, and start probing at 0x123E000, which is OK in this case.

[cuviper]

llvm/test/CodeGen/X86/stack-clash-small-large-align.ll

42 ↗

(On Diff #282221)

Oops, those start probing at 0x123F000, sorry for the bad math. The point stands though.

[serge-sans-paille]

Take into account @cuviper review, split handling of alignment between three cases:

1. small stack, small align → adjust first stack increase
2. large stack, small align → perform a first stack increase + touch before the loop
3. large align → perform a loop over the range between original stack pointer and aligned stack pointer (in that case, there's still a small window for the attack)

I still need to improve the third case...

[serge-sans-paille]

Perform masking and probing in the same location to nicely handle all combination of small/large alloc and small/large align

[serge-sans-paille]

Gentle ping :-)

[lkail]

LGTM. This solution probes the gap between [stackptr & mask, stackptr] when align is big.

[efriedma]

llvm/lib/Target/X86/X86FrameLowering.cpp

1097

I don't think this condition is right.

Say MaxAlign == StackProbeSize. Then an "and" can allocate up to StackProbeSize-4 bytes. So any subsequent stack allocation can jump over a guard page. (This is an extreme example. Really, it doesn't matter what the alignment is; it's just harder to cause a practical issue if the alignment is small.)

In general, we can't skip a probe for a stack allocation. We can only merge the probes for adjacent stack allocations. Say, for example, we realign the stack then allocate "Offset" bytes of aligned memory. We can get away with considering both allocations as a single "allocation" if MaxAlign+Offset <= StackProbeSize. But that method of proof works if you analyze them together. If you analyze each allocation independently, you can't prove the safety, so the realignment needs its own probe.

[lkail]

llvm/lib/Target/X86/X86FrameLowering.cpp

1097

Good catch. Looks PPC64's implementation also has the same issue. I'll post a patch to fix this issue for PPC64.

[serge-sans-paille]

Enforce an extra invariant: after alignment adjustment, there is less than PAGE_SIZE unprobed, which make sit possible to decouple it from stack allocation probing. I think this adresses @efriedma comment (?)

[efriedma]

I think this adresses @efriedma comment (?)

Maybe? I'd like to make sure we have test coverage for:

• the case where alignment+offset is less than the probe size
• the case where both alignment and offset are less than the probe size, but align+offset is greater than the probe size

(In stack-clash-small-alloc-medium-align.ll, it looks like alignment+offset is exactly equal to the probe size?)

[efriedma]

the case where alignment+offset is less than the probe size

For this case, it would also be good to have a version with a subsequent dynamic and/or callframe allocation that pushes the total stack allocated over the probe size, to show the interaction works correctly.

[serge-sans-paille]

Added more test case

[efriedma]

the case where both alignment and offset are less than the probe size, but align+offset is greater than the probe size

I don't think any of the tests cover this? Or am I missing something obvious?

[serge-sans-paille]

Added missing test case, and label existing ones so that their purpose is clear

[serge-sans-paille]

Tested clang bootstrapping with -fstack-clash-protection on, works like a charm \o/

[efriedma]

Looks like it's working correctly now. LGTM

llvm/lib/Target/X86/X86FrameLowering.h

void emitStackProbeInlineWindowsCoreCLR64(MachineFunction &MF,
                                             bool InProlog) const;
   void emitStackProbeInlineGeneric(MachineFunction &MF, MachineBasicBlock &MBB,
                                    MachineBasicBlock::iterator MBBI,
                                    const DebugLoc &DL, bool InProlog) const;
 
   void emitStackProbeInlineGenericBlock(MachineFunction &MF,
                                         MachineBasicBlock &MBB,
                                         MachineBasicBlock::iterator MBBI,
-                                        const DebugLoc &DL,
-                                        uint64_t Offset) const;
+                                        const DebugLoc &DL, uint64_t Offset,
+                                        uint64_t Align) const;
 
   void emitStackProbeInlineGenericLoop(MachineFunction &MF,
                                        MachineBasicBlock &MBB,
                                        MachineBasicBlock::iterator MBBI,
-                                       const DebugLoc &DL,
-                                       uint64_t Offset) const;
+                                       const DebugLoc &DL, uint64_t Offset,
+                                       uint64_t Align) const;
 
   /// Emit a stub to later inline the target stack probe.
   MachineInstr *emitStackProbeInlineStub(MachineFunction &MF,
                                          MachineBasicBlock &MBB,
                                          MachineBasicBlock::iterator MBBI,
                                          const DebugLoc &DL,
                                          bool InProlog) const;
 

llvm/lib/Target/X86/X86FrameLowering.cpp

void X86FrameLowering::emitStackProbeInlineGeneric(
   const X86Subtarget &STI = MF.getSubtarget<X86Subtarget>();
   const X86TargetLowering &TLI = *STI.getTargetLowering();
   assert(!(STI.is64Bit() && STI.isTargetWindowsCoreCLR()) &&
          "different expansion expected for CoreCLR 64 bit");
 
   const uint64_t StackProbeSize = TLI.getStackProbeSize(MF);
   uint64_t ProbeChunk = StackProbeSize * 8;
 
+  uint64_t MaxAlign = calculateMaxStackAlign(MF);
+
   // Synthesize a loop or unroll it, depending on the number of iterations.
-  if (Offset > ProbeChunk) {
-    emitStackProbeInlineGenericLoop(MF, MBB, MBBI, DL, Offset);
+  if (Offset > ProbeChunk || MaxAlign > ProbeChunk) {
+    emitStackProbeInlineGenericLoop(MF, MBB, MBBI, DL, Offset, MaxAlign);
   } else {
-    emitStackProbeInlineGenericBlock(MF, MBB, MBBI, DL, Offset);
+    emitStackProbeInlineGenericBlock(MF, MBB, MBBI, DL, Offset, MaxAlign);
   }
 }
 
 void X86FrameLowering::emitStackProbeInlineGenericBlock(
     MachineFunction &MF, MachineBasicBlock &MBB,
-    MachineBasicBlock::iterator MBBI, const DebugLoc &DL,
-    uint64_t Offset) const {
+    MachineBasicBlock::iterator MBBI, const DebugLoc &DL, uint64_t Offset,
+    uint64_t Align) const {
 
   const X86Subtarget &STI = MF.getSubtarget<X86Subtarget>();
   const X86TargetLowering &TLI = *STI.getTargetLowering();
   const unsigned Opc = getSUBriOpcode(Uses64BitFramePtr, Offset);
   const unsigned MovMIOpc = Is64Bit ? X86::MOV64mi32 : X86::MOV32mi;
   const uint64_t StackProbeSize = TLI.getStackProbeSize(MF);
+
   uint64_t CurrentOffset = 0;
-  // 0 Thanks to return address being saved on the stack
   uint64_t CurrentProbeOffset = 0;
 
+  if (CurrentOffset + StackProbeSize < Offset) {
+    assert(Align < StackProbeSize &&
+           "Should be an emitStackProbeInlineGenericLoop");
+
+    MachineInstr *MI = BuildMI(MBB, MBBI, DL, TII.get(Opc), StackPtr)
+                           .addReg(StackPtr)
+                           .addImm(StackProbeSize - Align)
+                           .setMIFlag(MachineInstr::FrameSetup);
+    MI->getOperand(3).setIsDead(); // The EFLAGS implicit def is dead.
+
+    addRegOffset(BuildMI(MBB, MBBI, DL, TII.get(MovMIOpc))
+                     .setMIFlag(MachineInstr::FrameSetup),
+                 StackPtr, false, 0)
+        .addImm(0)
+        .setMIFlag(MachineInstr::FrameSetup);
+    NumFrameExtraProbe++;
+    CurrentOffset = StackProbeSize - Align;
+  }
+
   // For the first N - 1 pages, just probe. I tried to take advantage of
   // natural probes but it implies much more logic and there was very few
   // interesting natural probes to interleave.
   while (CurrentOffset + StackProbeSize < Offset) {
     MachineInstr *MI = BuildMI(MBB, MBBI, DL, TII.get(Opc), StackPtr)
                            .addReg(StackPtr)
                            .addImm(StackProbeSize)
                            .setMIFlag(MachineInstr::FrameSetup);
MachineInstr *MI = BuildMI(MBB, MBBI, DL, TII.get(Opc), StackPtr)
                          .addReg(StackPtr)
                          .addImm(ChunkSize)
                          .setMIFlag(MachineInstr::FrameSetup);
   MI->getOperand(3).setIsDead(); // The EFLAGS implicit def is dead.
 }
 
 void X86FrameLowering::emitStackProbeInlineGenericLoop(
     MachineFunction &MF, MachineBasicBlock &MBB,
-    MachineBasicBlock::iterator MBBI, const DebugLoc &DL,
-    uint64_t Offset) const {
+    MachineBasicBlock::iterator MBBI, const DebugLoc &DL, uint64_t Offset,
+    uint64_t Align) const {
   assert(Offset && "null offset");
 
   const X86Subtarget &STI = MF.getSubtarget<X86Subtarget>();
   const X86TargetLowering &TLI = *STI.getTargetLowering();
   const unsigned MovMIOpc = Is64Bit ? X86::MOV64mi32 : X86::MOV32mi;
   const uint64_t StackProbeSize = TLI.getStackProbeSize(MF);
 
+  if (Align) {
+    const unsigned ADDOpc = getADDriOpcode(Uses64BitFramePtr, Align);
+    BuildMI(MBB, MBBI, DL, TII.get(ADDOpc), StackPtr)
+        .addReg(StackPtr)
+        .addImm(Align)
+        .setMIFlag(MachineInstr::FrameSetup);
+    Offset += Align;
+  }
+
   // Synthesize a loop
   NumFrameLoopProbe++;
   const BasicBlock *LLVM_BB = MBB.getBasicBlock();
 
   MachineBasicBlock *testMBB = MF.CreateMachineBasicBlock(LLVM_BB);
   MachineBasicBlock *tailMBB = MF.CreateMachineBasicBlock(LLVM_BB);
 
   MachineFunction::iterator MBBIter = ++MBB.getIterator();
   MF.insert(MBBIter, testMBB);
   MF.insert(MBBIter, tailMBB);
 
   Register FinalStackProbed = Uses64BitFramePtr ? X86::R11 : X86::R11D;
   BuildMI(MBB, MBBI, DL, TII.get(TargetOpcode::COPY), FinalStackProbed)
       .addReg(StackPtr)
       .setMIFlag(MachineInstr::FrameSetup);
 
   // save loop bound
   {
-    const unsigned Opc = getSUBriOpcode(Uses64BitFramePtr, Offset);
-    BuildMI(MBB, MBBI, DL, TII.get(Opc), FinalStackProbed)
+    const unsigned SUBOpc = getSUBriOpcode(Uses64BitFramePtr, Offset);
+    BuildMI(MBB, MBBI, DL, TII.get(SUBOpc), FinalStackProbed)
         .addReg(FinalStackProbed)
         .addImm(Offset / StackProbeSize * StackProbeSize)
         .setMIFlag(MachineInstr::FrameSetup);
   }
 
   // allocate a page
   {
-    const unsigned Opc = getSUBriOpcode(Uses64BitFramePtr, StackProbeSize);
-    BuildMI(testMBB, DL, TII.get(Opc), StackPtr)
+    const unsigned SUBOpc = getSUBriOpcode(Uses64BitFramePtr, StackProbeSize);
+    BuildMI(testMBB, DL, TII.get(SUBOpc), StackPtr)
         .addReg(StackPtr)
         .addImm(StackProbeSize)
         .setMIFlag(MachineInstr::FrameSetup);
   }
 
   // touch the page
   addRegOffset(BuildMI(testMBB, DL, TII.get(MovMIOpc))
                    .setMIFlag(MachineInstr::FrameSetup),
void X86FrameLowering::BuildStackAlignAND(MachineBasicBlock &MBB,
 
   // The EFLAGS implicit def is dead.
   MI->getOperand(3).setIsDead();
 }
 
 bool X86FrameLowering::has128ByteRedZone(const MachineFunction& MF) const {
   // x86-64 (non Win64) has a 128 byte red zone which is guaranteed not to be
   // clobbered by any interrupt handler.
   assert(&STI == &MF.getSubtarget<X86Subtarget>() &&
          "MF used frame lowering for wrong subtarget");
   const Function &Fn = MF.getFunction();
   const bool IsWin64CC = STI.isCallingConvWin64(Fn.getCallingConv());
   return Is64Bit && !IsWin64CC && !Fn.hasFnAttribute(Attribute::NoRedZone);
 }
 
 
 /// emitPrologue - Push callee-saved registers onto the stack, which

llvm/test/CodeGen/X86/stack-clash-large.ll

define i32 @foo() local_unnamed_addr #0 {
   %c = load volatile i32, i32* %a
   ret i32 %c
 }
 
 attributes #0 =  {"probe-stack"="inline-asm"}
 
 ; CHECK-X86-64-LABEL: foo:
 ; CHECK-X86-64:        # %bb.0:
+; CHECK-X86-64-NEXT:  addq $16, %rsp
 ; CHECK-X86-64-NEXT:  movq  %rsp, %r11
 ; CHECK-X86-64-NEXT:  subq  $69632, %r11 # imm = 0x11000
 ; CHECK-X86-64-NEXT:   .LBB0_1:
 ; CHECK-X86-64-NEXT:  subq  $4096, %rsp # imm = 0x1000
 ; CHECK-X86-64-NEXT:  movq  $0, (%rsp)
 ; CHECK-X86-64-NEXT:  cmpq  %r11, %rsp
 ; CHECK-X86-64-NEXT:  jne .LBB0_1
 ; CHECK-X86-64-NEXT:# %bb.2:
-; CHECK-X86-64-NEXT:  subq    $2248, %rsp
+; CHECK-X86-64-NEXT:  subq    $2264, %rsp
 ; CHECK-X86-64-NEXT:  .cfi_def_cfa_offset 71888
 ; CHECK-X86-64-NEXT:  movl  $1, 264(%rsp)
 ; CHECK-X86-64-NEXT:  movl  $1, 28664(%rsp)
 ; CHECK-X86-64-NEXT:  movl  -128(%rsp), %eax
 ; CHECK-X86-64-NEXT:  addq  $71880, %rsp # imm = 0x118C8
 ; CHECK-X86-64-NEXT:  .cfi_def_cfa_offset 8
 ; CHECK-X86-64-NEXT:  retq
 
 ; CHECK-X86-32-LABEL: foo:
 ; CHECK-X86-32:      # %bb.0:
+; CHECK-X86-32-NEXT:  addl $16, %esp
 ; CHECK-X86-32-NEXT:    movl    %esp, %r11d
 ; CHECK-X86-32-NEXT:    subl    $69632, %r11d # imm = 0x11000
 ; CHECK-X86-32-NEXT:    .LBB0_1: # =>This Inner Loop Header: Depth=1
 ; CHECK-X86-32-NEXT:    subl    $4096, %esp # imm = 0x1000
 ; CHECK-X86-32-NEXT:    movl    $0, (%esp)
 ; CHECK-X86-32-NEXT:    cmpl    %r11d, %esp
 ; CHECK-X86-32-NEXT:    jne  .LBB0_1
 ; CHECK-X86-32-NEXT:# %bb.2:
-; CHECK-X86-32-NEXT:    subl    $2380, %esp
+; CHECK-X86-32-NEXT:    subl    $2396, %esp
 ; CHECK-X86-32-NEXT:    .cfi_def_cfa_offset 72016
 ; CHECK-X86-32-NEXT:    movl    $1, 392(%esp)
 ; CHECK-X86-32-NEXT:    movl    $1, 28792(%esp)
 ; CHECK-X86-32-NEXT:    movl    (%esp), %eax
 ; CHECK-X86-32-NEXT:    addl    $72012, %esp # imm = 0x1194C
 ; CHECK-X86-32-NEXT:    .cfi_def_cfa_offset 4
 ; CHECK-X86-32-NEXT:    retl

llvm/test/CodeGen/X86/stack-clash-medium-natural-probes-mutliple-objects.ll

 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
 ; RUN: llc < %s | FileCheck %s
 
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
 
 define i32 @foo() local_unnamed_addr #0 {
 ; CHECK-LABEL: foo:
 ; CHECK:       # %bb.0:
-; CHECK-NEXT:    subq $4096, %rsp # imm = 0x1000
+; CHECK-NEXT:    subq $4080, %rsp # imm = 0xFF0
 ; CHECK-NEXT:    movq $0, (%rsp)
-; CHECK-NEXT:    subq $1784, %rsp # imm = 0x6F8
+; CHECK-NEXT:    subq $1800, %rsp # imm = 0x708
 ; CHECK-NEXT:    .cfi_def_cfa_offset 5888
 ; CHECK-NEXT:    movl $1, {{[0-9]+}}(%rsp)
 ; CHECK-NEXT:    movl $2, {{[0-9]+}}(%rsp)
 ; CHECK-NEXT:    movl {{[0-9]+}}(%rsp), %eax
 ; CHECK-NEXT:    addq $5880, %rsp # imm = 0x16F8
 ; CHECK-NEXT:    .cfi_def_cfa_offset 8
 ; CHECK-NEXT:    retq
   %a = alloca i32, i64 1000, align 16

llvm/test/CodeGen/X86/stack-clash-medium-natural-probes.ll

 ; RUN: llc < %s | FileCheck %s
 
 
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
 
 define i32 @foo() local_unnamed_addr #0 {
 
 ; CHECK-LABEL: foo:
 ; CHECK:         # %bb.0:
-; CHECK-NEXT: subq  $4096, %rsp             # imm = 0x1000
+; CHECK-NEXT: subq  $4080, %rsp             # imm = 0xFF0
 ; CHECK-NEXT: movq  $0, (%rsp)
-; CHECK-NEXT: subq  $3784, %rsp             # imm = 0xEC8
+; CHECK-NEXT: subq  $3800, %rsp             # imm = 0xED8
 ; CHECK-NEXT: .cfi_def_cfa_offset 7888
 ; CHECK-NEXT: movl  $1, 264(%rsp)
 ; CHECK-NEXT: movl  $1, 4664(%rsp)
 ; CHECK-NEXT: movl  -128(%rsp), %eax
 ; CHECK-NEXT: addq  $7880, %rsp             # imm = 0x1EC8
 ; CHECK-NEXT: .cfi_def_cfa_offset 8
 ; CHECK-NEXT: retq
 

llvm/test/CodeGen/X86/stack-clash-medium.ll

 ; RUN: llc -mtriple=x86_64-linux-android < %s | FileCheck -check-prefix=CHECK-X86-64 %s 
 ; RUN: llc -mtriple=i686-linux-android < %s | FileCheck -check-prefix=CHECK-X86-32 %s 
 
 define i32 @foo() local_unnamed_addr #0 {
   %a = alloca i32, i64 2000, align 16
   %b = getelementptr inbounds i32, i32* %a, i64 200
   store volatile i32 1, i32* %b
   %c = load volatile i32, i32* %a
   ret i32 %c
 }
 
 attributes #0 =  {"probe-stack"="inline-asm"}
 
 ; CHECK-X86-64-LABEL: foo:
 ; CHECK-X86-64:      # %bb.0:
-; CHECK-X86-64-NEXT: subq $4096, %rsp             # imm = 0x1000
+; CHECK-X86-64-NEXT: subq $4080, %rsp             # imm = 0xFF0
 ; CHECK-X86-64-NEXT: movq $0, (%rsp)
-; CHECK-X86-64-NEXT: subq $3784, %rsp             # imm = 0xEC8
+; CHECK-X86-64-NEXT: subq $3800, %rsp             # imm = 0xED8
 ; CHECK-X86-64-NEXT: .cfi_def_cfa_offset 7888
 ; CHECK-X86-64-NEXT: movl $1, 672(%rsp)
 ; CHECK-X86-64-NEXT: movl -128(%rsp), %eax
 ; CHECK-X86-64-NEXT: addq $7880, %rsp             # imm = 0x1EC8
 ; CHECK-X86-64-NEXT: .cfi_def_cfa_offset 8
 ; CHECK-X86-64-NEXT: retq
 
 
 ; CHECK-X86-32-LABEL: foo:
 ; CHECK-X86-32:      # %bb.0:
-; CHECK-X86-32-NEXT: subl $4096, %esp # imm = 0x1000
+; CHECK-X86-32-NEXT: subl $4080, %esp # imm = 0xFF0
 ; CHECK-X86-32-NEXT: movl $0, (%esp)
-; CHECK-X86-32-NEXT: subl $3916, %esp # imm = 0xF4C
+; CHECK-X86-32-NEXT: subl $3932, %esp # imm = 0xF5C
 ; CHECK-X86-32-NEXT: .cfi_def_cfa_offset 8016
 ; CHECK-X86-32-NEXT: movl $1, 800(%esp)
 ; CHECK-X86-32-NEXT: movl (%esp), %eax
 ; CHECK-X86-32-NEXT: addl $8012, %esp # imm = 0x1F4C
 ; CHECK-X86-32-NEXT: .cfi_def_cfa_offset 4
 ; CHECK-X86-32-NEXT: retl

llvm/test/CodeGen/X86/stack-clash-no-free-probe.ll

 ; RUN: llc < %s | FileCheck %s
 
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
 
 define i32 @foo(i64 %i) local_unnamed_addr #0 {
 ; CHECK-LABEL: foo:
 ; CHECK:       # %bb.0:
-; CHECK-NEXT:  subq $4096, %rsp # imm = 0x1000
+; CHECK-NEXT:  subq $4080, %rsp # imm = 0xFF0
 ; CHECK-NEXT:  movq $0, (%rsp)
-; CHECK-NEXT:  subq $3784, %rsp # imm = 0xEC8
+; CHECK-NEXT:  subq $3800, %rsp # imm = 0xED8
 ; CHECK-NEXT:  .cfi_def_cfa_offset 7888
 ; CHECK-NEXT:  movl $1, -128(%rsp,%rdi,4)
 ; CHECK-NEXT:  movl -128(%rsp), %eax
 ; CHECK-NEXT:  addq $7880, %rsp # imm = 0x1EC8
 ; CHECK-NEXT:  .cfi_def_cfa_offset 8
 ; CHECK-NEXT:  retq
 
   %a = alloca i32, i32 2000, align 16
   %b = getelementptr inbounds i32, i32* %a, i64 %i
   store volatile i32 1, i32* %b
   %c = load volatile i32, i32* %a
   ret i32 %c
 }
 
 attributes #0 =  {"probe-stack"="inline-asm"}
 

llvm/test/CodeGen/X86/stack-clash-unknown-call.ll

 ; RUN: llc < %s | FileCheck %s
 
 
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
 
 declare void @llvm.memset.p0i8.i64(i8* nocapture writeonly, i8, i64, i1 immarg);
 
 define void @foo() local_unnamed_addr #0 {
 
 ;CHECK-LABEL: foo:
 ;CHECK:         # %bb.0:
-;CHECK-NEXT:  subq  $4096, %rsp # imm = 0x1000
+;CHECK-NEXT:  subq  $4080, %rsp # imm = 0xFF0
 ; it's important that we don't use the call as a probe here
 ;CHECK-NEXT:  movq  $0, (%rsp)
-;CHECK-NEXT:  subq  $3912, %rsp # imm = 0xF48
+;CHECK-NEXT:  subq  $3928, %rsp # imm = 0xF58
 ;CHECK-NEXT:  .cfi_def_cfa_offset 8016
 ;CHECK-NEXT:  movq  %rsp, %rdi
 ;CHECK-NEXT:  movl  $8000, %edx # imm = 0x1F40
 ;CHECK-NEXT:  xorl  %esi, %esi
 ;CHECK-NEXT:  callq memset
 ;CHECK-NEXT:  addq  $8008, %rsp # imm = 0x1F48
 ;CHECK-NEXT:  .cfi_def_cfa_offset 8
 ;CHECK-NEXT:  retq
 
   %a = alloca i8, i64 8000, align 16
   call void @llvm.memset.p0i8.i64(i8* align 16 %a, i8 0, i64 8000, i1 false)
   ret void
 }
 
 attributes #0 =  {"probe-stack"="inline-asm"}