[llvm] [StackColoring] Handle SEH catch object stack slots conservatively (PR #66988)
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Thu Sep 21 00:35:45 PDT 2023
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-backend-x86
<details>
<summary>Changes</summary>
The write to the SEH catch object happens before cleanuppads are executed, while the first reference to the object will typically be in a catchpad.
If we make use of first-use analysis, we may end up allocating an alloca used inside the cleanuppad and the catch object at the same stack offset, which would be incorrect.
https://reviews.llvm.org/D86673 was a previous attempt to fix it. It used the heuristic "a slot loaded in a WinEH pad and never written" to detect catch objects. However, because it checks for more than one load (while probably more than zero was intended), the fix does not actually work.
The general approach also seems dubious to me, so this patch reverts that change entirely, and instead marks all catch object slots as conservative (i.e. excluded from first-use analysis) based on the WinEHFuncInfo. As far as I can tell we don't need any heuristics here, we know exactly which slots are affected.
Fixes https://github.com/llvm/llvm-project/issues/66984.
---
Full diff: https://github.com/llvm/llvm-project/pull/66988.diff
2 Files Affected:
- (modified) llvm/lib/CodeGen/StackColoring.cpp (+13-48)
- (modified) llvm/test/CodeGen/X86/stack-coloring-seh.ll (+4-4)
``````````diff
diff --git a/llvm/lib/CodeGen/StackColoring.cpp b/llvm/lib/CodeGen/StackColoring.cpp
index 4d69b6104f68372..d90c3e95f477944 100644
--- a/llvm/lib/CodeGen/StackColoring.cpp
+++ b/llvm/lib/CodeGen/StackColoring.cpp
@@ -370,37 +370,6 @@ STATISTIC(EscapedAllocas, "Number of allocas that escaped the lifetime region");
// If in RPO ordering chosen to walk the CFG we happen to visit the b[k]
// before visiting the memcpy block (which will contain the lifetime start
// for "b" then it will appear that 'b' has a degenerate lifetime.
-//
-// Handle Windows Exception with LifetimeStartOnFirstUse:
-// -----------------
-//
-// There was a bug for using LifetimeStartOnFirstUse in win32.
-// class Type1 {
-// ...
-// ~Type1(){ write memory;}
-// }
-// ...
-// try{
-// Type1 V
-// ...
-// } catch (Type2 X){
-// ...
-// }
-// For variable X in catch(X), we put point pX=&(&X) into ConservativeSlots
-// to prevent using LifetimeStartOnFirstUse. Because pX may merged with
-// object V which may call destructor after implicitly writing pX. All these
-// are done in C++ EH runtime libs (through CxxThrowException), and can't
-// obviously check it in IR level.
-//
-// The loader of pX, without obvious writing IR, is usually the first LOAD MI
-// in EHPad, Some like:
-// bb.x.catch.i (landing-pad, ehfunclet-entry):
-// ; predecessors: %bb...
-// successors: %bb...
-// %n:gr32 = MOV32rm %stack.pX ...
-// ...
-// The Type2** %stack.pX will only be written in EH runtime libs, so we
-// check the StoreSlots to screen it out.
namespace {
@@ -462,9 +431,6 @@ class StackColoring : public MachineFunctionPass {
/// slots lifetime-start-on-first-use is disabled).
BitVector ConservativeSlots;
- /// Record the FI slots referenced by a 'may write to memory'.
- BitVector StoreSlots;
-
/// Number of iterations taken during data flow analysis.
unsigned NumIterations;
@@ -660,13 +626,10 @@ unsigned StackColoring::collectMarkers(unsigned NumSlot) {
InterestingSlots.resize(NumSlot);
ConservativeSlots.clear();
ConservativeSlots.resize(NumSlot);
- StoreSlots.clear();
- StoreSlots.resize(NumSlot);
// number of start and end lifetime ops for each slot
SmallVector<int, 8> NumStartLifetimes(NumSlot, 0);
SmallVector<int, 8> NumEndLifetimes(NumSlot, 0);
- SmallVector<int, 8> NumLoadInCatchPad(NumSlot, 0);
// Step 1: collect markers and populate the "InterestingSlots"
// and "ConservativeSlots" sets.
@@ -722,13 +685,6 @@ unsigned StackColoring::collectMarkers(unsigned NumSlot) {
if (! BetweenStartEnd.test(Slot)) {
ConservativeSlots.set(Slot);
}
- // Here we check the StoreSlots to screen catch point out. For more
- // information, please refer "Handle Windows Exception with
- // LifetimeStartOnFirstUse" at the head of this file.
- if (MI.mayStore())
- StoreSlots.set(Slot);
- if (MF->getWinEHFuncInfo() && MBB->isEHPad() && MI.mayLoad())
- NumLoadInCatchPad[Slot] += 1;
}
}
}
@@ -739,14 +695,23 @@ unsigned StackColoring::collectMarkers(unsigned NumSlot) {
return 0;
}
- // 1) PR27903: slots with multiple start or end lifetime ops are not
+ // PR27903: slots with multiple start or end lifetime ops are not
// safe to enable for "lifetime-start-on-first-use".
- // 2) And also not safe for variable X in catch(X) in windows.
for (unsigned slot = 0; slot < NumSlot; ++slot) {
- if (NumStartLifetimes[slot] > 1 || NumEndLifetimes[slot] > 1 ||
- (NumLoadInCatchPad[slot] > 1 && !StoreSlots.test(slot)))
+ if (NumStartLifetimes[slot] > 1 || NumEndLifetimes[slot] > 1)
ConservativeSlots.set(slot);
}
+
+ // The write to the catch object by the personality function is not propely
+ // modeled in IR: It happens before any cleanuppads are executed, even if the
+ // first mention of the catch object is in a catchpad. As such, mark catch
+ // object slots as conservative, so they are excluded from first-use analysis.
+ if (WinEHFuncInfo *EHInfo = MF->getWinEHFuncInfo())
+ for (WinEHTryBlockMapEntry &TBME : EHInfo->TryBlockMap)
+ for (WinEHHandlerType &H : TBME.HandlerArray)
+ if (H.CatchObj.FrameIndex != std::numeric_limits<int>::max())
+ ConservativeSlots.set(H.CatchObj.FrameIndex);
+
LLVM_DEBUG(dumpBV("Conservative slots", ConservativeSlots));
// Step 2: compute begin/end sets for each block
diff --git a/llvm/test/CodeGen/X86/stack-coloring-seh.ll b/llvm/test/CodeGen/X86/stack-coloring-seh.ll
index 3995bfa8101d69d..892c81a12dc1acf 100644
--- a/llvm/test/CodeGen/X86/stack-coloring-seh.ll
+++ b/llvm/test/CodeGen/X86/stack-coloring-seh.ll
@@ -3,7 +3,7 @@
@type_info = external global ptr
-; FIXME: This is a miscompile.
+; Make sure %a1 and %a2 don't share the same stack offset.
define void @pr66984(ptr %arg) personality ptr @__CxxFrameHandler3 {
; CHECK-LABEL: pr66984:
; CHECK: # %bb.0: # %bb
@@ -12,7 +12,7 @@ define void @pr66984(ptr %arg) personality ptr @__CxxFrameHandler3 {
; CHECK-NEXT: pushl %ebx
; CHECK-NEXT: pushl %edi
; CHECK-NEXT: pushl %esi
-; CHECK-NEXT: subl $20, %esp
+; CHECK-NEXT: subl $24, %esp
; CHECK-NEXT: movl %esp, -28(%ebp)
; CHECK-NEXT: movl $-1, -16(%ebp)
; CHECK-NEXT: leal -24(%ebp), %eax
@@ -31,7 +31,7 @@ define void @pr66984(ptr %arg) personality ptr @__CxxFrameHandler3 {
; CHECK-NEXT: $ehgcr_0_4:
; CHECK-NEXT: movl -24(%ebp), %eax
; CHECK-NEXT: movl %eax, %fs:0
-; CHECK-NEXT: addl $20, %esp
+; CHECK-NEXT: addl $24, %esp
; CHECK-NEXT: popl %esi
; CHECK-NEXT: popl %edi
; CHECK-NEXT: popl %ebx
@@ -47,7 +47,7 @@ define void @pr66984(ptr %arg) personality ptr @__CxxFrameHandler3 {
; CHECK-NEXT: pushl %ebp
; CHECK-NEXT: addl $12, %ebp
; CHECK-NEXT: movl %esp, -28(%ebp)
-; CHECK-NEXT: movl -32(%ebp), %ecx
+; CHECK-NEXT: movl -36(%ebp), %ecx
; CHECK-NEXT: movl $2, -16(%ebp)
; CHECK-NEXT: calll _cleanup
; CHECK-NEXT: movl $LBB0_3, %eax
``````````
</details>
https://github.com/llvm/llvm-project/pull/66988
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