[win][x64] Re-use fixed object if multiple catchpads use the same alloca for their catch objects (#147849)

Addresses
<#147421 (comment)>
for x86

If more than one `catchpad ` uses the same `alloca` for their catch
objects, then we will allocate more than one object in the fixed area
resulting in wasted stack space.

As a follow up, Clang could be updated to re-use the same `alloca` for
all by-reference and by-pointer catch objects.

Author: dpaoliello

llvm/lib/Target/X86/X86FrameLowering.cpp

@@ -4241,7 +4241,7 @@ void X86FrameLowering::adjustFrameForMsvcCxxEh(MachineFunction &MF) const {
   for (WinEHTryBlockMapEntry &TBME : EHInfo.TryBlockMap) {
     for (WinEHHandlerType &H : TBME.HandlerArray) {
       int FrameIndex = H.CatchObj.FrameIndex;
-      if (FrameIndex != INT_MAX) {
+      if ((FrameIndex != INT_MAX) && MFI.getObjectOffset(FrameIndex) == 0) {
         // Ensure alignment.
         unsigned Align = MFI.getObjectAlign(FrameIndex).value();
         MinFixedObjOffset -= std::abs(MinFixedObjOffset) % Align;

llvm/test/CodeGen/WinEH/wineh-reuse-catch-alloca.ll

@@ -0,0 +1,100 @@
+; RUN: llc %s --mtriple=x86_64-pc-windows-msvc -o - | FileCheck %s
+
+; Tests the fixed object layouts when two catchpads re-use the same stack
+; allocation for this catch objects.
+
+; Generated from this C++ code, with modifications to the IR (see comments in
+; IR):
+; https://godbolt.org/z/9qv5Yn68j
+; > clang --target=x86_64-pc-windows-msvc test.cpp
+; ```
+; extern "C" void boom();
+; extern "C" int calls_boom();
+; {
+;     try { boom(); }
+;     catch (int& i) { return i; }
+;     catch (long& l) { return l; }
+;     return 0;
+; }
+; ```
+
+; Minimum stack alloc is 64 bytes, so no change there.
+; CHECK-LABEL:  calls_boom:
+; CHECK:        subq    $64, %rsp
+; CHECK:        .seh_stackalloc 64
+
+; Both the catch blocks load from the same address.
+; CHECK-LABEL:  "?catch$3@?0?calls_boom@4HA":
+; CHECK:        movq    -8(%rbp), %rax
+; CHECK-LABEL:  "?catch$4@?0?calls_boom@4HA":
+; CHECK:        movq    -8(%rbp), %rax
+
+; There's enough space for the UnwindHelp to be at 48 instead of 40
+; CHECK-LABEL:  $cppxdata$calls_boom:
+; CHECK:        .long   48                              # UnwindHelp
+
+; Both catches have the same object offset.
+; CHECK-LABEL:  $handlerMap$0$calls_boom:
+; CHECK:        .long   56                              # CatchObjOffset
+; CHECK-NEXT:   .long   "?catch$3@?0?calls_boom@4HA"@IMGREL # Handler
+; CHECK:        .long   56                              # CatchObjOffset
+; CHECK-NEXT:   .long   "?catch$4@?0?calls_boom@4HA"@IMGREL # Handler
+
+%rtti.TypeDescriptor2 = type { ptr, ptr, [3 x i8] }
+
+$"??_R0H@8" = comdat any
+
+$"??_R0J@8" = comdat any
+
+@"??_7type_info@@6B@" = external constant ptr
+@"??_R0H@8" = linkonce_odr global %rtti.TypeDescriptor2 { ptr @"??_7type_info@@6B@", ptr null, [3 x i8] c".H\00" }, comdat
+@"??_R0J@8" = linkonce_odr global %rtti.TypeDescriptor2 { ptr @"??_7type_info@@6B@", ptr null, [3 x i8] c".J\00" }, comdat
+
+define dso_local i32 @calls_boom() personality ptr @__CxxFrameHandler3 {
+entry:
+  %retval = alloca i32, align 4
+; MODIFICATION: Remove unusued alloca
+;  %l = alloca ptr, align 8
+  %i = alloca ptr, align 8
+  invoke void @boom()
+          to label %invoke.cont unwind label %catch.dispatch
+
+catch.dispatch:
+  %0 = catchswitch within none [label %catch1, label %catch] unwind to caller
+
+catch1:
+  %1 = catchpad within %0 [ptr @"??_R0H@8", i32 8, ptr %i]
+  %2 = load ptr, ptr %i, align 8
+  %3 = load i32, ptr %2, align 4
+  store i32 %3, ptr %retval, align 4
+  catchret from %1 to label %catchret.dest2
+
+catch:
+; MODIFICATION: Use %i instead of %l
+  %4 = catchpad within %0 [ptr @"??_R0J@8", i32 8, ptr %i]
+  %5 = load ptr, ptr %i, align 8
+  %6 = load i32, ptr %5, align 4
+  store i32 %6, ptr %retval, align 4
+  catchret from %4 to label %catchret.dest
+
+invoke.cont:
+  br label %try.cont
+
+catchret.dest:
+  br label %return
+
+catchret.dest2:
+  br label %return
+
+try.cont:
+  store i32 0, ptr %retval, align 4
+  br label %return
+
+return:
+  %7 = load i32, ptr %retval, align 4
+  ret i32 %7
+}
+
+declare dso_local void @boom() #1
+
+declare dso_local i32 @__CxxFrameHandler3(...)