Fix bug in llvm::DemoteRegToStack

[ahatanak]

Summary

In the following example, when DemoteRegToStack demotes %call.i.i.i14.i.i, it firsts scans all the uses of %call.i.i.i14.i.i and inserts load instructions before all its users. Since %0 is a phi instruction, a load instruction is inserted in the predecessor block, which is the entry basic block. Then it inserts a store instruction to save the value it is demoting to the stack. In this case, %call.i.i.i14.i.i is an invoke, so it creates a basic block that splits the critical edge (entry-do.body.i.i.i), and inserts the store there. This is incorrect because the store instruction should be executed before the load instruction.

define void @_Z4foo1c(i8 signext %a) {
entry:

...
%call.i.i.i14.i.i = invoke noalias i8* @_Znwm(i32 1024)
        to label %do.body.i.i.i unwind label %lpad.body

do.body.i.i.i: ; preds = %entry, %_ZNSt3__116allocator_traitsINS_9allocatorIcEEE9constructIccEEvRS2_PT_RKT0_.exit.i.i.i

%0 = phi i8* [ %incdec.ptr.i.i.i, %_ZNSt3__116allocator_traitsINS_9allocatorIcEEE9constructIccEEvRS2_PT_RKT0_.exit.i.i.i ], [ %call.i.i.i14.i.i, %entry ]
...

The IR after SjLjEHPrepare pass looks like this:

define void @_Z4foo1c(i8 signext %a) {
entry:

%call.i.i.i14.i.i.reg2mem = alloca i8*
 ...
%call.i.i.i14.i.i.reload = load volatile i8** %call.i.i.i14.i.i.reg2mem
...
%call.i.i.i14.i.i = invoke noalias i8* @_Znwm(i32 1024)
        to label %entry.do.body.i.i.i_crit_edge unwind label %lpad.body

entry.do.body.i.i.i_crit_edge: ; preds = %entry

store i8* %call.i.i.i14.i.i, i8** %call.i.i.i14.i.i.reg2mem
br label %do.body.i.i.i

do.body.i.i.i: ; preds = %entry.do.body.i.i.i_crit_edge, %_ZNSt3__116allocator_traitsINS_9allocatorIcEEE9constructIccEEvRS2_PT_RKT0_.exit.i.i.i

%3 = phi i8* [ %incdec.ptr.i.i.i, %_ZNSt3__116allocator_traitsINS_9allocatorIcEEE9constructIccEEvRS2_PT_RKT0_.exit.i.i.i ], [ %call.i.i.i14.i.i.reload, %entry.do.body.i.i.i_crit_edge ]

This patch fixes the bug by changing llvm::DemoteRegToStack to insert the store instruction and create basic blocks that split critical edges first, and then insert the instructions to reload the value.

[nicholas]

• lib/Transforms/Utils/DemoteRegToStack.cpp

+++ lib/Transforms/Utils/DemoteRegToStack.cpp
@@ -21,6 +21,7 @@

/// alloca.  This allows the CFG to be changed around without fear of
/// invalidating the SSA information for the value.  It returns the

pointer to

/// the alloca inserted to create a stack slot for I.

+

AllocaInst *llvm::DemoteRegToStack(Instruction &I, bool VolatileLoads,
                                   Instruction *AllocaPoint) {
  if (I.use_empty()) {

Remove the extra blank line, this comment is attached to this function.

+ StoreInst *Store = new StoreInst(&I, Slot, InsertPt);
+
+ Change all of the users of the instruction, except the Store we've
just
+ inserted, to read from the stack slot.
+ SmallVector<User*, 4> UsersOfI(I.users());
+
+ for (User *U : UsersOfI) {
+ if (PHINode *PN = dyn_cast<PHINode>(U)) {

Can you avoid the extra copy? What if you create the store now, but
don't insert it yet? (I think InsertPt can't be one of the users, correct?)

While we're here:

for (; isa<PHINode>(InsertPt) || isa<LandingPadInst>(InsertPt);

++InsertPt)

/* empty */;   // Don't insert before PHI nodes or landingpad instrs.

This is redundant with getFirstInsertionPt. Can this be hoisted up to
the only place that sets InsertPt without using getFirstInsertionPt?

Nick

Akira Hatanaka wrote:

In the following example, when DemoteRegToStack demotes %call.i.i.i14.i.i, it firsts scans all the uses of %call.i.i.i14.i.i and inserts load instructions before all its users. Since %0 is a phi instruction, a load instruction is inserted in the predecessor block, which is the entry basic block. Then it inserts a store instruction to save the value it is demoting to the stack. In this case, %call.i.i.i14.i.i is an invoke, so it creates a basic block that splits the critical edge (entry-do.body.i.i.i), and inserts the store there. This is incorrect because the store instruction should be executed before the load instruction.

define void @_Z4foo1c(i8 signext %a) {
entry:

...
%call.i.i.i14.i.i = invoke noalias i8* @_Znwm(i32 1024)
        to label %do.body.i.i.i unwind label %lpad.body

do.body.i.i.i: ; preds = %entry, %_ZNSt3__116allocator_traitsINS_9allocatorIcEEE9constructIccEEvRS2_PT_RKT0_.exit.i.i.i

%0 = phi i8* [ %incdec.ptr.i.i.i, %_ZNSt3__116allocator_traitsINS_9allocatorIcEEE9constructIccEEvRS2_PT_RKT0_.exit.i.i.i ], [ %call.i.i.i14.i.i, %entry ]
...

The IR after SjLjEHPrepare pass looks like this:

define void @_Z4foo1c(i8 signext %a) {
entry:

%call.i.i.i14.i.i.reg2mem = alloca i8*
 ...
%call.i.i.i14.i.i.reload = load volatile i8** %call.i.i.i14.i.i.reg2mem
...
%call.i.i.i14.i.i = invoke noalias i8* @_Znwm(i32 1024)
        to label %entry.do.body.i.i.i_crit_edge unwind label %lpad.body

entry.do.body.i.i.i_crit_edge: ; preds = %entry

store i8* %call.i.i.i14.i.i, i8** %call.i.i.i14.i.i.reg2mem
br label %do.body.i.i.i

do.body.i.i.i: ; preds = %entry.do.body.i.i.i_crit_edge, %_ZNSt3__116allocator_traitsINS_9allocatorIcEEE9constructIccEEvRS2_PT_RKT0_.exit.i.i.i

%3 = phi i8* [ %incdec.ptr.i.i.i, %_ZNSt3__116allocator_traitsINS_9allocatorIcEEE9constructIccEEvRS2_PT_RKT0_.exit.i.i.i ], [ %call.i.i.i14.i.i.reload, %entry.do.body.i.i.i_crit_edge ]

This patch fixes the bug by changing llvm::DemoteRegToStack to insert the store instruction and create basic blocks that split critical edges first, and then insert the instructions to reload the value.

http://reviews.llvm.org/D6729

Files:

lib/Transforms/Utils/DemoteRegToStack.cpp
test/CodeGen/ARM/sjlj-prepare-critical-edge.ll

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---

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[ahatanak]

+ StoreInst *Store = new StoreInst(&I, Slot, InsertPt);
+
+ Change all of the users of the instruction, except the Store we've
just
+ inserted, to read from the stack slot.
+ SmallVector<User*, 4> UsersOfI(I.users());
+
+ for (User *U : UsersOfI) {
+ if (PHINode *PN = dyn_cast<PHINode>(U)) {

Can you avoid the extra copy? What if you create the store now, but
don't insert it yet? (I think InsertPt can't be one of the users, correct?)

I'm probably not understanding your suggestion, but is it possible to create the Store instruction without adding it the UseList of "I"? Also, I think InsertPt can be one of the users if the def (instruction "I") is immediately followed by a user of "I".

If that doesn't work, I was thinking maybe I can just split it into two parts. The first part just splits the critical edge and fixes the CFG, if that's necessary. After that, load instructions are inserted before all the users. The last step computes the insertion point and inserts the store instruction there.

[nicholas]

Akira Hatanaka wrote:

+ StoreInst *Store = new StoreInst(&I, Slot, InsertPt);

+

+  // Change all of the users of the instruction, except the Store we've

just

+  // inserted, to read from the stack slot.

+  SmallVector<User*, 4>  UsersOfI(I.users());

+

+  for (User *U : UsersOfI) {

+    if (PHINode *PN = dyn_cast<PHINode>(U)) {

Can you avoid the extra copy? What if you create the store now, but

don't insert it yet? (I think InsertPt can't be one of the users, correct?)

I'm probably not understanding your suggestion, but is it possible to create the Store instruction without adding it the UseList of "I"?

Yes. "StoreInst *SI = net StoreInst(Val, Ptr);" will create it without
inserting it anywhere, then you can insert it later via.
SI->insertBefore(Inst) or insertAfter(Inst).

Also, I think InsertPt can be one of the users if the def (instruction
"I") is immediately followed by a user of "I".

If that doesn't work, I was thinking maybe I can just split it into two parts. The first part just splits the critical edge and fixes the CFG, if that's necessary. After that, load instructions are inserted before all the users. The last step computes the insertion point and inserts the store instruction there.

http://reviews.llvm.org/D6729

EMAIL PREFERENCES

http://reviews.llvm.org/settings/panel/emailpreferences/

[ahatanak]

Patch updated.

I moved the piece of code which splits the critical edge before the while loop which replaces all the user of I. I believe this should work, although this is probably different from the solution you suggested. Please let me know if I completely misunderstood your comments.

I didn't quite understand your comment about creating the store instruction and inserting it later. You can create the store without inserting it into the basic block, but you are still adding the store to the use list of instruction "I", no? If so, the while loop which replaces all the uses of "I" has to replace the value operand of the store (because the loop won't terminate otherwise), which is not what we want to do.

Also, in the case where the use immediately follows the def, I think the insertion point of the store has to be recomputed after the use is replaced and the load is inserted (I'm assuming it's legitimate to use DemoteRegToStack in such case).

1. before transformation.

v0 = Def
Use v0 <- store's insertPt

2. after replacing use of v0.

v0 = Def
v1 = load stackslot
Use v1 <- store's insertPt

3. after inserting store.

v0 = Def
v1 = load stackslot
store v0, stackslot <- this should be inserted before the load
Use v1

[ahatanak]

Added a line to silence possible warning from release builds.

Nick, do you have further comments?

[nicholas]

Akira Hatanaka wrote:

Added a line to silence possible warning from release builds.

Nick, do you have further comments?

Nope! LGTM

[ahatanak]

Thanks, I'll commit this patch shortly.

lib/Transforms/Utils/DemoteRegToStack.cpp

if (AllocaPoint) {
     Slot = new AllocaInst(I.getType(), nullptr,
                           I.getName()+".reg2mem", AllocaPoint);
   } else {
     Function *F = I.getParent()->getParent();
     Slot = new AllocaInst(I.getType(), nullptr, I.getName()+".reg2mem",
                           F->getEntryBlock().begin());
   }
 
+  // We cannot demote invoke instructions to the stack if their normal edge
+  // is critical. Therefore, split the critical edge and create a basic block
+  // into which the store can be inserted.
+  if (InvokeInst *II = dyn_cast<InvokeInst>(&I)) {
+    if (!II->getNormalDest()->getSinglePredecessor()) {
+      unsigned SuccNum = GetSuccessorNumber(II->getParent(), II->getNormalDest());
+      assert(isCriticalEdge(II, SuccNum) && "Expected a critical edge!");
+      BasicBlock *BB = SplitCriticalEdge(II, SuccNum);
+      assert(BB && "Unable to split critical edge.");
+      (void)BB;
+    }
+  }
+
   // Change all of the users of the instruction to read from the stack slot.
   while (!I.use_empty()) {
     Instruction *U = cast<Instruction>(I.user_back());
     if (PHINode *PN = dyn_cast<PHINode>(U)) {
       // If this is a PHI node, we can't insert a load of the value before the
       // use.  Instead insert the load in the predecessor block corresponding
       // to the incoming value.
       //
while (!I.use_empty()) {
 
     } else {
       // If this is a normal instruction, just insert a load.
       Value *V = new LoadInst(Slot, I.getName()+".reload", VolatileLoads, U);
       U->replaceUsesOfWith(&I, V);
     }
   }
 
-
   // Insert stores of the computed value into the stack slot. We have to be
   // careful if I is an invoke instruction, because we can't insert the store
   // AFTER the terminator instruction.
   BasicBlock::iterator InsertPt;
   if (!isa<TerminatorInst>(I)) {
     InsertPt = &I;
     ++InsertPt;
+    for (; isa<PHINode>(InsertPt) || isa<LandingPadInst>(InsertPt); ++InsertPt)
+      /* empty */;   // Don't insert before PHI nodes or landingpad instrs.
   } else {
     InvokeInst &II = cast<InvokeInst>(I);
-    if (II.getNormalDest()->getSinglePredecessor())
-      InsertPt = II.getNormalDest()->getFirstInsertionPt();
-    else {
-      // We cannot demote invoke instructions to the stack if their normal edge
-      // is critical.  Therefore, split the critical edge and insert the store
-      // in the newly created basic block.
-      unsigned SuccNum = GetSuccessorNumber(I.getParent(), II.getNormalDest());
-      TerminatorInst *TI = &cast<TerminatorInst>(I);
-      assert (isCriticalEdge(TI, SuccNum) &&
-              "Expected a critical edge!");
-      BasicBlock *BB = SplitCriticalEdge(TI, SuccNum);
-      assert (BB && "Unable to split critical edge.");
-      InsertPt = BB->getFirstInsertionPt();
-    }
   }
 
-  for (; isa<PHINode>(InsertPt) || isa<LandingPadInst>(InsertPt); ++InsertPt)
-    /* empty */;   // Don't insert before PHI nodes or landingpad instrs.
-
   new StoreInst(&I, Slot, InsertPt);
   return Slot;
 }
 
 /// DemotePHIToStack - This function takes a virtual register computed by a PHI
 /// node and replaces it with a slot in the stack frame allocated via alloca.
 /// The PHI node is deleted. It returns the pointer to the alloca inserted.
 AllocaInst *llvm::DemotePHIToStack(PHINode *P, Instruction *AllocaPoint) {

test/CodeGen/ARM/sjlj-prepare-critical-edge.ll

-; RUN: llc < %s -O1 -mtriple thumbv7-apple-ios6
+; RUN: llc < %s -O1 -mtriple thumbv7-apple-ios6 | FileCheck %s
 ; Just make sure no one tries to make the assumption that the normal edge of an
 ; invoke is never a critical edge.  Previously, this code would assert.
 
 %struct.__CFString = type opaque
 
 declare void @bar(%struct.__CFString*, %struct.__CFString*)
 
 define noalias i8* @foo(i8* nocapture %inRefURL) noreturn ssp {

 
 declare i8* @bar2()
 
 declare i32 @__gxx_personality_sj0(...)
 
 declare void @release(i8*)
 
 declare void @terminatev()
+
+; Make sure that the instruction DemoteRegToStack inserts to reload
+; %call.i.i.i14.i.i follows the instruction that saves the value to the stack in
+; basic block %entry.do.body.i.i.i_crit_edge.
+; Previously, DemoteRegToStack would insert a load instruction into the entry
+; block to reload %call.i.i.i14.i.i before the phi instruction (%0) in block
+; %do.body.i.i.i.
+
+; CHECK-LABEL: __Z4foo1c:
+; CHECK: blx __Znwm
+; CHECK: {{.*}}@ %entry.do.body.i.i.i_crit_edge
+; CHECK: str r0, [sp, [[OFFSET:#[0-9]+]]]
+; CHECK: ldr [[R0:r[0-9]+]], [sp, [[OFFSET]]]
+; CHECK: {{.*}}@ %do.body.i.i.i
+; CHECK: cmp [[R0]], #0
+
+%"class.std::__1::basic_string" = type { %"class.std::__1::__compressed_pair" }
+%"class.std::__1::__compressed_pair" = type { %"class.std::__1::__libcpp_compressed_pair_imp" }
+%"class.std::__1::__libcpp_compressed_pair_imp" = type { %"struct.std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >::__rep" }
+%"struct.std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >::__rep" = type { %union.anon }
+%union.anon = type { %"struct.std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >::__long" }
+%"struct.std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >::__long" = type { i32, i32, i8* }
+
+@.str = private unnamed_addr constant [12 x i8] c"some_string\00", align 1
+
+define void @_Z4foo1c(i8 signext %a) {
+entry:
+  %s1 = alloca %"class.std::__1::basic_string", align 4
+  call void @_ZNSt3__112basic_stringIcNS_11char_traitsIcEENS_9allocatorIcEEE6__initEPKcm(%"class.std::__1::basic_string"* %s1, i8* getelementptr inbounds ([12 x i8]* @.str, i32 0, i32 0), i32 11)
+  %call.i.i.i14.i.i = invoke noalias i8* @_Znwm(i32 1024)
+          to label %do.body.i.i.i unwind label %lpad.body
+
+do.body.i.i.i:                                    ; preds = %entry, %_ZNSt3__116allocator_traitsINS_9allocatorIcEEE9constructIccEEvRS2_PT_RKT0_.exit.i.i.i
+  %lsr.iv = phi i32 [ %lsr.iv.next, %_ZNSt3__116allocator_traitsINS_9allocatorIcEEE9constructIccEEvRS2_PT_RKT0_.exit.i.i.i ], [ -1024, %entry ]
+  %0 = phi i8* [ %incdec.ptr.i.i.i, %_ZNSt3__116allocator_traitsINS_9allocatorIcEEE9constructIccEEvRS2_PT_RKT0_.exit.i.i.i ], [ %call.i.i.i14.i.i, %entry ]
+  %new.isnull.i.i.i.i = icmp eq i8* %0, null
+  br i1 %new.isnull.i.i.i.i, label %_ZNSt3__116allocator_traitsINS_9allocatorIcEEE9constructIccEEvRS2_PT_RKT0_.exit.i.i.i, label %new.notnull.i.i.i.i
+
+new.notnull.i.i.i.i:                              ; preds = %do.body.i.i.i
+  store i8 %a, i8* %0, align 1
+  br label %_ZNSt3__116allocator_traitsINS_9allocatorIcEEE9constructIccEEvRS2_PT_RKT0_.exit.i.i.i
+
+_ZNSt3__116allocator_traitsINS_9allocatorIcEEE9constructIccEEvRS2_PT_RKT0_.exit.i.i.i: ; preds = %new.notnull.i.i.i.i, %do.body.i.i.i
+  %1 = phi i8* [ null, %do.body.i.i.i ], [ %0, %new.notnull.i.i.i.i ]
+  %incdec.ptr.i.i.i = getelementptr inbounds i8* %1, i32 1
+  %lsr.iv.next = add i32 %lsr.iv, 1
+  %cmp.i16.i.i = icmp eq i32 %lsr.iv.next, 0
+  br i1 %cmp.i16.i.i, label %invoke.cont, label %do.body.i.i.i
+
+invoke.cont:                                      ; preds = %_ZNSt3__116allocator_traitsINS_9allocatorIcEEE9constructIccEEvRS2_PT_RKT0_.exit.i.i.i
+  invoke void @_Z4foo2Pci(i8* %call.i.i.i14.i.i, i32 1024)
+          to label %invoke.cont5 unwind label %lpad2
+
+invoke.cont5:                                     ; preds = %invoke.cont
+  %cmp.i.i.i15 = icmp eq i8* %call.i.i.i14.i.i, null
+  br i1 %cmp.i.i.i15, label %invoke.cont6, label %_ZNSt3__113__vector_baseIcNS_9allocatorIcEEE5clearEv.exit.i.i.i19
+
+_ZNSt3__113__vector_baseIcNS_9allocatorIcEEE5clearEv.exit.i.i.i19: ; preds = %invoke.cont5
+  call void @_ZdlPv(i8* %call.i.i.i14.i.i)
+  br label %invoke.cont6
+
+invoke.cont6:                                     ; preds = %_ZNSt3__113__vector_baseIcNS_9allocatorIcEEE5clearEv.exit.i.i.i19, %invoke.cont5
+  %call10 = call %"class.std::__1::basic_string"* @_ZNSt3__112basic_stringIcNS_11char_traitsIcEENS_9allocatorIcEEED1Ev(%"class.std::__1::basic_string"* %s1)
+  ret void
+
+lpad.body:                                        ; preds = %entry
+  %2 = landingpad { i8*, i32 } personality i8* bitcast (i32 (...)* @__gxx_personality_sj0 to i8*)
+          cleanup
+  %3 = extractvalue { i8*, i32 } %2, 0
+  %4 = extractvalue { i8*, i32 } %2, 1
+  br label %ehcleanup
+
+lpad2:                                            ; preds = %invoke.cont
+  %5 = landingpad { i8*, i32 } personality i8* bitcast (i32 (...)* @__gxx_personality_sj0 to i8*)
+          cleanup
+  %6 = extractvalue { i8*, i32 } %5, 0
+  %7 = extractvalue { i8*, i32 } %5, 1
+  %cmp.i.i.i21 = icmp eq i8* %call.i.i.i14.i.i, null
+  br i1 %cmp.i.i.i21, label %ehcleanup, label %_ZNSt3__113__vector_baseIcNS_9allocatorIcEEE5clearEv.exit.i.i.i26
+
+_ZNSt3__113__vector_baseIcNS_9allocatorIcEEE5clearEv.exit.i.i.i26: ; preds = %lpad2
+  call void @_ZdlPv(i8* %call.i.i.i14.i.i)
+  br label %ehcleanup
+
+ehcleanup:                                        ; preds = %_ZNSt3__113__vector_baseIcNS_9allocatorIcEEE5clearEv.exit.i.i.i26, %lpad2, %lpad.body
+  %exn.slot.0 = phi i8* [ %3, %lpad.body ], [ %6, %lpad2 ], [ %6, %_ZNSt3__113__vector_baseIcNS_9allocatorIcEEE5clearEv.exit.i.i.i26 ]
+  %ehselector.slot.0 = phi i32 [ %4, %lpad.body ], [ %7, %lpad2 ], [ %7, %_ZNSt3__113__vector_baseIcNS_9allocatorIcEEE5clearEv.exit.i.i.i26 ]
+  %call12 = invoke %"class.std::__1::basic_string"* @_ZNSt3__112basic_stringIcNS_11char_traitsIcEENS_9allocatorIcEEED1Ev(%"class.std::__1::basic_string"* %s1)
+          to label %eh.resume unwind label %terminate.lpad
+
+eh.resume:                                        ; preds = %ehcleanup
+  %lpad.val = insertvalue { i8*, i32 } undef, i8* %exn.slot.0, 0
+  %lpad.val13 = insertvalue { i8*, i32 } %lpad.val, i32 %ehselector.slot.0, 1
+  resume { i8*, i32 } %lpad.val13
+
+terminate.lpad:                                   ; preds = %ehcleanup
+  %8 = landingpad { i8*, i32 } personality i8* bitcast (i32 (...)* @__gxx_personality_sj0 to i8*)
+          catch i8* null
+  %9 = extractvalue { i8*, i32 } %8, 0
+  call void @__clang_call_terminate(i8* %9)
+  unreachable
+}
+
+declare void @_Z4foo2Pci(i8*, i32)
+
+define linkonce_odr hidden void @__clang_call_terminate(i8*) {
+  %2 = tail call i8* @__cxa_begin_catch(i8* %0)
+  tail call void @_ZSt9terminatev()
+  unreachable
+}
+
+declare i8* @__cxa_begin_catch(i8*)
+declare void @_ZSt9terminatev()
+declare %"class.std::__1::basic_string"* @_ZNSt3__112basic_stringIcNS_11char_traitsIcEENS_9allocatorIcEEED1Ev(%"class.std::__1::basic_string"* returned)
+declare void @_ZdlPv(i8*) #3
+declare noalias i8* @_Znwm(i32)
+declare void @_ZNSt3__112basic_stringIcNS_11char_traitsIcEENS_9allocatorIcEEE6__initEPKcm(%"class.std::__1::basic_string"*, i8*, i32)
+declare void @_Unwind_SjLj_Register({ i8*, i32, [4 x i32], i8*, i8*, [5 x i8*] }*)
+declare void @_Unwind_SjLj_Unregister({ i8*, i32, [4 x i32], i8*, i8*, [5 x i8*] }*)
+declare i8* @llvm.frameaddress(i32)
+declare i8* @llvm.stacksave()
+declare void @llvm.stackrestore(i8*)
+declare i32 @llvm.eh.sjlj.setjmp(i8*)
+declare i8* @llvm.eh.sjlj.lsda()
+declare void @llvm.eh.sjlj.callsite(i32)
+declare void @llvm.eh.sjlj.functioncontext(i8*)