[X86] When optimizing for size, use POP for small post-call stack clean-up

[mkuper]

Summary

When optimizing for size, replace "addl $4, %esp" and "addl $8, %esp" following a call by one or two pops, respectively.

This patch doesn't try to do it in general, but only when the stack adjustment immediately follows a call - which is the most common case.
That allows taking a short-cut when trying to find a free register to pop into, instead of a full-blown liveness check. If the adjustment immediately follows a call, then every register the call clobbers but doesn't define should be dead at that point, and can be used.

[majnemer]

lib/Target/X86/X86FrameLowering.cpp

1857–1858

Is PopSize interestingly different from SlotSize?

1875

SmallVector seems like a little overkill, would an array of type unsigned [2] make more sense?

1880–1881

Can this be a range-based for loop?

1908–1910

Why bother looking for two free registers if we can count on being able to reuse the first one? Does it cause a false dependency or something? More curiosity than anything else.

[mkuper]

Thanks, David!

Do you see any issues with the way I'm choosing the registers?
I need a sanity check on the "if it's clobbered and not imp-def'ed, it's free" thing.

lib/Target/X86/X86FrameLowering.cpp

1857–1858

No, should be the same. :)

1875

Force of habit more than anything else, will change to an array if you prefer that.

1880–1881

Yes.

1908–1910

I'm afraid of a false dependency, yes. I haven't actually measured it.

[mkuper]

Updated with David's comments.

Honestly, I liked the SmallVector version better. Are you ok with reverting to that?

[mkuper]

Ping

[rnk]

lgtm

In D11749#217500, @mkuper wrote:

Updated with David's comments.

Honestly, I liked the SmallVector version better. Are you ok with reverting to that?

I'd rather not. If the STL accessors make things more readable, how about std::array?

test/CodeGen/X86/pop-stack-cleanup.ll

4

Add a test that returns i64 in EAX:EDX and see what we get.

[mkuper]

std::array doesn't really help readability, I think.
Will keep it as is, then.

Also, on second thought, I'd rather be safe and only enable this for minsize, not optsize, for now.

lib/Target/X86/X86FrameLowering.h

private:
   void BuildCFI(MachineBasicBlock &MBB, MachineBasicBlock::iterator MBBI,
                 DebugLoc DL, MCCFIInstruction CFIInst) const;
 
   /// Aligns the stack pointer by ANDing it with -MaxAlign.
   void BuildStackAlignAND(MachineBasicBlock &MBB,
                           MachineBasicBlock::iterator MBBI, DebugLoc DL,
                           uint64_t MaxAlign) const;
 
+  /// Make small positive stack adjustments using POPs.
+  bool adjustStackWithPops(MachineBasicBlock &MBB,
+                           MachineBasicBlock::iterator MBBI, DebugLoc DL,
+                           int Offset) const;
+
   /// Adjusts the stack pointer using LEA, SUB, or ADD.
   MachineInstrBuilder BuildStackAdjustment(MachineBasicBlock &MBB,
                                            MachineBasicBlock::iterator MBBI,
                                            DebugLoc DL, int64_t Offset,
                                            bool InEpilogue) const;
 };
 
 } // End llvm namespace
 
 #endif

lib/Target/X86/X86FrameLowering.cpp

if (MaxStack > Guaranteed) {
     incStackMBB->addSuccessor(&PrologueMBB, 99);
     incStackMBB->addSuccessor(incStackMBB, 1);
   }
 #ifdef XDEBUG
   MF.verify();
 #endif
 }
 
+bool X86FrameLowering::adjustStackWithPops(MachineBasicBlock &MBB,
+    MachineBasicBlock::iterator MBBI, DebugLoc DL, int Offset) const {
+
+  if (Offset % SlotSize)
+    return false;
+
+  int NumPops = Offset / SlotSize;
+  // This is only worth it if we have at most 2 pops.
+  if (NumPops != 1 && NumPops != 2)
+    return false;
+
+  // Handle only the trivial case where the adjustment directly follows
+  // a call. This is the most common one, anyway.
+  if (MBBI == MBB.begin())
+    return false;
+  MachineBasicBlock::iterator Prev = std::prev(MBBI);
+  if (!Prev->isCall() || !Prev->getOperand(1).isRegMask())
+    return false;
+
+  unsigned Regs[2];
+  unsigned FoundRegs = 0;
+
+  auto RegMask = Prev->getOperand(1);
+  
+  // Try to find up to NumPops free registers. 
+  for (auto Candidate : X86::GR32_NOREX_NOSPRegClass) {
+
+    // Poor man's liveness:
+    // Since we're immediately after a call, any register that is clobbered
+    // by the call and not defined by it can be considered dead.
+    if (!RegMask.clobbersPhysReg(Candidate))
+      continue;
+
+    bool IsDef = false;
+    for (const MachineOperand &MO : Prev->implicit_operands()) {
+      if (MO.isReg() && MO.isDef() && MO.getReg() == Candidate) {
+        IsDef = true;
+        break;
+      }
+    }
+
+    if (IsDef)
+      continue;
+
+    Regs[FoundRegs++] = Candidate;
+    if (FoundRegs == (unsigned)NumPops)
+      break;
+  }
+
+  if (FoundRegs == 0)
+    return false;
+
+  // If we found only one free register, but need two, reuse the same one twice.
+  while (FoundRegs < (unsigned)NumPops)
+    Regs[FoundRegs++] = Regs[0];
+
+  for (int i = 0; i < NumPops; ++i)
+    BuildMI(MBB, MBBI, DL, 
+            TII.get(STI.is64Bit() ? X86::POP64r : X86::POP32r), Regs[i]);
+
+  return true;
+}
+
 void X86FrameLowering::
 eliminateCallFramePseudoInstr(MachineFunction &MF, MachineBasicBlock &MBB,
                               MachineBasicBlock::iterator I) const {
   bool reserveCallFrame = hasReservedCallFrame(MF);
   unsigned Opcode = I->getOpcode();
   bool isDestroy = Opcode == TII.getCallFrameDestroyOpcode();
   DebugLoc DL = I->getDebugLoc();
   uint64_t Amount = !reserveCallFrame ? I->getOperand(0).getImm() : 0;
if (!reserveCallFrame) {
 
     // Factor out the amount that gets handled inside the sequence
     // (Pushes of argument for frame setup, callee pops for frame destroy)
     Amount -= InternalAmt;
 
     if (Amount) {
       // Add Amount to SP to destroy a frame, and subtract to setup.
       int Offset = isDestroy ? Amount : -Amount;
+
+      const Function *F = MF.getFunction();
+      bool OptForSize = F->hasFnAttribute(Attribute::OptimizeForSize) ||
+                        F->hasFnAttribute(Attribute::MinSize);
+
+      if (!(OptForSize && adjustStackWithPops(MBB, I, DL, Offset)))
         BuildStackAdjustment(MBB, I, DL, Offset, /*InEpilogue=*/false);
     }
+
     return;
   }
 
   if (isDestroy && InternalAmt) {
     // If we are performing frame pointer elimination and if the callee pops
     // something off the stack pointer, add it back.  We do this until we have
     // more advanced stack pointer tracking ability.
     // We are not tracking the stack pointer adjustment by the callee, so make

test/CodeGen/X86/fold-push.ll

 ; RUN: llc < %s -mtriple=i686-windows | FileCheck %s -check-prefix=CHECK -check-prefix=NORMAL
 ; RUN: llc < %s -mtriple=i686-windows -mattr=call-reg-indirect | FileCheck %s -check-prefix=CHECK -check-prefix=SLM
 
 declare void @foo(i32 %r)
 
 define void @test(i32 %a, i32 %b) optsize {
 ; CHECK-LABEL: test:
 ; CHECK: movl [[EAX:%e..]], (%esp)
 ; CHECK-NEXT: pushl [[EAX]]
 ; CHECK-NEXT: calll
-; CHECK-NEXT: addl $4, %esp
+; CHECK-NEXT: popl
 ; CHECK: nop
 ; NORMAL: pushl (%esp)
 ; SLM: movl (%esp), [[RELOAD:%e..]]
 ; SLM-NEXT: pushl [[RELOAD]]
 ; CHECK: calll
-; CHECK-NEXT: addl $4, %esp
+; CHECK-NEXT: popl
   %c = add i32 %a, %b
   call void @foo(i32 %c)
   call void asm sideeffect "nop", "~{ax},~{bx},~{cx},~{dx},~{bp},~{si},~{di}"()
   call void @foo(i32 %c)
   ret void
 }
 
 define void @test_min(i32 %a, i32 %b) minsize {
 ; CHECK-LABEL: test_min:
 ; CHECK: movl [[EAX:%e..]], (%esp)
 ; CHECK-NEXT: pushl [[EAX]]
 ; CHECK-NEXT: calll
-; CHECK-NEXT: addl $4, %esp
+; CHECK-NEXT: popl
 ; CHECK: nop
 ; CHECK: pushl (%esp)
 ; CHECK: calll
-; CHECK-NEXT: addl $4, %esp
+; CHECK-NEXT: popl
   %c = add i32 %a, %b
   call void @foo(i32 %c)
   call void asm sideeffect "nop", "~{ax},~{bx},~{cx},~{dx},~{bp},~{si},~{di}"()
   call void @foo(i32 %c)
   ret void
 }

test/CodeGen/X86/pop-stack-cleanup.ll

+; RUN: llc < %s -mtriple=i686-windows | FileCheck %s -check-prefix=CHECK -check-prefix=NORMAL
+
+declare void @param1(i32 %a)
+declare i32 @param2_ret(i32 %a, i32 %b)
+declare void @param2(i32 %a, i32 %b)
+declare void @param3(i32 %a, i32 %b, i32 %c)
+
+define void @test() optsize {
+; CHECK-LABEL: test:
+; CHECK: calll _param1
+; CHECK-NEXT: popl %eax
+; CHECK: calll _param2
+; CHECK-NEXT: popl %eax
+; CHECK-NEXT: popl %ecx
+; CHECK: calll _param2_ret
+; CHECK-NEXT: popl %ecx
+; CHECK-NEXT: popl %edx
+; CHECK-NEXT: pushl %eax
+; CHECK: calll _param3
+; CHECK-NEXT: addl $12, %esp
+  call void @param1(i32 1)
+  call void @param2(i32 1, i32 2)
+  %ret = call i32 @param2_ret(i32 1, i32 2)
+  call void @param3(i32 1, i32 2, i32 %ret)
+  ret void
+}
+
+define void @negative(i32 %k) {
+; CHECK-LABEL: negative:
+; CHECK: calll _param1
+; CHECK-NEXT: addl $4, %esp
+; CHECK: calll _param2
+; CHECK-NEXT: addl $8, %esp
+; CHECK: calll _param3
+; CHECK-NEXT: movl %ebp, %esp
+  %v = alloca i32, i32 %k
+  call void @param1(i32 1)
+  call void @param2(i32 1, i32 2)
+  call void @param3(i32 1, i32 2, i32 3)
+  ret void
+}
+
+define void @spill(i32 inreg %a, i32 inreg %b, i32 inreg %c) optsize {
+; CHECK-LABEL: spill:
+; CHECK-DAG: movl %ecx,
+; CHECK-DAG: movl %edx,
+; CHECK: calll _param2_ret
+; CHECK-NEXT: popl %ecx
+; CHECK-NEXT: popl %edx
+; CHECK-DAG: movl {{.*}}, %ecx
+; CHECK-DAG: movl {{.*}}, %edx
+; CHECK: calll _spill
+  %i = call i32 @param2_ret(i32 1, i32 2)
+  call void @spill(i32 %a, i32 %b, i32 %c)
+  ret void
+}