[win][x64] Allow push/pop for stack alloc when unwind v2 is required (#153621)

While attempting to enable Windows x64 unwind v2, compilation failed
with the following error:

```
fatal error: error in backend: Windows x64 Unwind v2 is required, but LLVM has generated incompatible code in function '<redacted>': Cannot pop registers before the stack allocation has been deallocated
```

I traced this down to an optimization in `X86FrameLowering`:

<https://github.com/llvm/llvm-project/blob/6961139ce9154d03c88b8d46c8742a1eaa569cd9/llvm/lib/Target/X86/X86FrameLowering.cpp#L324-L340>

Technically, using `push`/`pop` to adjust the stack is permitted under
unwind v2: the requirement for a "canonical" epilog is that the stack is
fully adjusted before the registers listed as pushed in the unwind table
are popped. So, as long as the `.seh_unwindv2start` pseudo is after the
pops that adjust the stack, then everything will work correctly.

One other side effect of this change is that the stack is now allowed to
be adjusted across multiple instructions, which would be needed for
extremely large stack frames.

Author: dpaoliello

llvm/lib/Target/X86/X86WinEHUnwindV2.cpp

@@ -201,15 +201,11 @@ bool X86WinEHUnwindV2::runOnMachineFunction(MachineFunction &MF) {
                 "The epilog is deallocating a stack "
                 "allocation, but the prolog did "
                 "not allocate one");
-          if (HasStackDealloc)
+          if (PoppedRegCount > 0)
             return rejectCurrentFunctionInternalError(
                 MF, Mode,
-                "The epilog is deallocating the stack "
-                "allocation more than once");
-          if (PoppedRegCount > 0)
-            llvm_unreachable(
-                "Should have raised an error: either popping before "
-                "deallocating or deallocating without an allocation");
+                "The epilog is deallocating a stack allocation after popping "
+                "registers");
 
           HasStackDealloc = true;
         } else if (State == FunctionState::FinishedEpilog)
@@ -219,33 +215,41 @@ bool X86WinEHUnwindV2::runOnMachineFunction(MachineFunction &MF) {
 
       case X86::POP64r:
         if (State == FunctionState::InEpilog) {
-          // After the stack pointer has been adjusted, the epilog must
-          // POP each register in reverse order of the PUSHes in the prolog.
-          PoppedRegCount++;
-          if (HasStackAlloc != HasStackDealloc)
-            return rejectCurrentFunctionInternalError(
-                MF, Mode,
-                "Cannot pop registers before the stack "
-                "allocation has been deallocated");
-          if (PoppedRegCount > PushedRegs.size())
-            return rejectCurrentFunctionInternalError(
-                MF, Mode,
-                "The epilog is popping more registers than the prolog pushed");
-          if (PushedRegs[PushedRegs.size() - PoppedRegCount] !=
-              MI.getOperand(0).getReg())
-            return rejectCurrentFunctionInternalError(
-                MF, Mode,
-                "The epilog is popping a registers in "
-                "a different order than the "
-                "prolog pushed them");
-
-          // Unwind v2 records the size of the epilog not from where we place
-          // SEH_BeginEpilogue (as that contains the instruction to adjust the
-          // stack pointer) but from the first POP instruction (if there is
-          // one).
-          if (!UnwindV2StartLocation) {
-            assert(PoppedRegCount == 1);
-            UnwindV2StartLocation = &MI;
+          Register Reg = MI.getOperand(0).getReg();
+          if (HasStackAlloc && (PoppedRegCount == 0) &&
+              !llvm::is_contained(PushedRegs, Reg)) {
+            // If this is a pop that doesn't correspond to the set of pushed
+            // registers, then assume it was used to adjust the stack pointer.
+            HasStackDealloc = true;
+          } else {
+            // After the stack pointer has been adjusted, the epilog must
+            // POP each register in reverse order of the PUSHes in the prolog.
+            PoppedRegCount++;
+            if (HasStackAlloc != HasStackDealloc)
+              return rejectCurrentFunctionInternalError(
+                  MF, Mode,
+                  "Cannot pop registers before the stack "
+                  "allocation has been deallocated");
+            if (PoppedRegCount > PushedRegs.size())
+              return rejectCurrentFunctionInternalError(
+                  MF, Mode,
+                  "The epilog is popping more registers than the prolog "
+                  "pushed");
+            if (PushedRegs[PushedRegs.size() - PoppedRegCount] != Reg)
+              return rejectCurrentFunctionInternalError(
+                  MF, Mode,
+                  "The epilog is popping a registers in "
+                  "a different order than the "
+                  "prolog pushed them");
+
+            // Unwind v2 records the size of the epilog not from where we place
+            // SEH_BeginEpilogue (as that contains the instruction to adjust the
+            // stack pointer) but from the first POP instruction (if there is
+            // one).
+            if (!UnwindV2StartLocation) {
+              assert(PoppedRegCount == 1);
+              UnwindV2StartLocation = &MI;
+            }
           }
         } else if (State == FunctionState::FinishedEpilog)
           // Unexpected instruction after the epilog.

llvm/test/CodeGen/X86/win64-eh-unwindv2-errors.mir

@@ -97,38 +97,6 @@ body:             |
     RET64
 ...
 
-;--- double_dealloc.mir
-# RUN: not --crash llc -mtriple=x86_64-pc-windows-msvc -o - %t/double_dealloc.mir \
-# RUN:    -run-pass=x86-wineh-unwindv2 2>&1 | FileCheck %s \
-# RUN:    --check-prefix=DOUBLE-DEALLOC
-# RUN: llc -mtriple=x86_64-pc-windows-msvc -o - %t/double_dealloc.mir \
-# RUN:    -run-pass=x86-wineh-unwindv2 -x86-wineh-unwindv2-force-mode=1 | \
-# RUN:    FileCheck %s --check-prefix=BESTEFFORT
-# DOUBLE-DEALLOC: LLVM ERROR: Windows x64 Unwind v2 is required, but LLVM has generated incompatible code in function 'double_dealloc':
-# DOUBLE-DEALLOC-SAME: The epilog is deallocating the stack allocation more than once
-
---- |
-  define dso_local void @double_dealloc() local_unnamed_addr {
-  entry:
-    ret void
-  }
-  !llvm.module.flags = !{!0}
-  !0 = !{i32 1, !"winx64-eh-unwindv2", i32 2}
-...
----
-name:            double_dealloc
-body:             |
-  bb.0.entry:
-    $rsp = frame-setup SUB64ri32 $rsp, 40, implicit-def dead $eflags
-    frame-setup SEH_StackAlloc 40
-    frame-setup SEH_EndPrologue
-    SEH_BeginEpilogue
-    $rsp = frame-destroy ADD64ri32 $rsp, 40, implicit-def dead $eflags
-    $rsp = frame-destroy ADD64ri32 $rsp, 40, implicit-def dead $eflags
-    SEH_EndEpilogue
-    RET64
-...
-
 ;--- dealloc_after_epilog.mir
 # RUN: not --crash llc -mtriple=x86_64-pc-windows-msvc -o - \
 # RUN:    %t/dealloc_after_epilog.mir -run-pass=x86-wineh-unwindv2 2>&1 | \
@@ -316,3 +284,38 @@ body:             |
     $ecx = MOV32rr killed $eax
     RET64
 ...
+
+;--- dealloc_pop_dealloc.mir
+# RUN: not --crash llc -mtriple=x86_64-pc-windows-msvc -o - \
+# RUN:    %t/dealloc_pop_dealloc.mir -run-pass=x86-wineh-unwindv2 2>&1 | \
+# RUN:    FileCheck %s --check-prefix=DEALLOC-POP-DEALLOC
+# RUN: llc -mtriple=x86_64-pc-windows-msvc -o - %t/dealloc_pop_dealloc.mir \
+# RUN:    -run-pass=x86-wineh-unwindv2 -x86-wineh-unwindv2-force-mode=1 | \
+# RUN:    FileCheck %s --check-prefix=BESTEFFORT
+# DEALLOC-POP-DEALLOC: LLVM ERROR: Windows x64 Unwind v2 is required, but LLVM has generated incompatible code in function 'dealloc_pop_dealloc':
+# DEALLOC-POP-DEALLOC-SAME: The epilog is deallocating a stack allocation after popping registers
+
+--- |
+  define dso_local void @dealloc_pop_dealloc() local_unnamed_addr {
+  entry:
+    ret void
+  }
+  !llvm.module.flags = !{!0}
+  !0 = !{i32 1, !"winx64-eh-unwindv2", i32 2}
+...
+---
+name:            dealloc_pop_dealloc
+body:             |
+  bb.0.entry:
+    frame-setup PUSH64r killed $rdi, implicit-def $rsp, implicit $rsp
+    frame-setup SEH_PushReg 55
+    $rsp = frame-setup SUB64ri32 $rsp, 40, implicit-def dead $eflags
+    frame-setup SEH_StackAlloc 40
+    frame-setup SEH_EndPrologue
+    SEH_BeginEpilogue
+    $rsp = frame-destroy ADD64ri32 $rsp, 20, implicit-def dead $eflags
+    $rdi = frame-destroy POP64r implicit-def $rsp, implicit $rsp
+    $rsp = frame-destroy ADD64ri32 $rsp, 20, implicit-def dead $eflags
+    SEH_EndEpilogue
+    RET64
+...

llvm/test/CodeGen/X86/win64-eh-unwindv2-push-pop-stack-alloc.mir

@@ -0,0 +1,199 @@
+# RUN: llc -o - %s -mtriple=x86_64-unknown-windows-msvc \
+# RUN:  -run-pass=x86-wineh-unwindv2 | FileCheck %s
+
+# Regression test for Win x64 unwind v2: in some cases it is better to use
+# push+pop to adjust the stack, rather than sub+add. This is permitted with
+# unwind v2 as the requirement is that the epilog finishes adjusting the stack
+# before popping the registers listed in the unwind table.
+
+# Pushes and pops the same register.
+# CHECK-LABEL:  name:            push_pop_same
+# CHECK:        body:
+# CHECK-NEXT:     bb.0
+# CHECK-NEXT:       SEH_UnwindVersion 2
+# CHECK-NEXT:       frame-setup PUSH64r undef $rax
+# CHECK-NEXT:       frame-setup SEH_StackAlloc 8
+# CHECK-NEXT:       frame-setup SEH_EndPrologue
+# CHECK-NEXT:       SEH_BeginEpilogue
+# CHECK-NEXT:       $rax = frame-destroy
+# CHECK-NEXT:       SEH_UnwindV2Start
+# CHECK-NEXT:       SEH_EndEpilogue
+# CHECK-NEXT:       RET64
+
+# Pushes and pops a different register.
+# CHECK-LABEL:  name:            push_pop_different
+# CHECK:        body:
+# CHECK-NEXT:     bb.0
+# CHECK-NEXT:       SEH_UnwindVersion 2
+# CHECK-NEXT:       frame-setup PUSH64r undef $rax
+# CHECK-NEXT:       frame-setup SEH_StackAlloc 8
+# CHECK-NEXT:       frame-setup SEH_EndPrologue
+# CHECK:            SEH_BeginEpilogue
+# CHECK-NEXT:       $rcx = frame-destroy POP64r
+# CHECK-NEXT:       SEH_UnwindV2Start
+# CHECK-NEXT:       SEH_EndEpilogue
+# CHECK-NEXT:       RET64 $eax
+
+# Pushes in the prolog, adds in the epilog.
+# CHECK-LABEL:  name:            push_add
+# CHECK:        body:
+# CHECK-NEXT:     bb.0
+# CHECK-NEXT:       SEH_UnwindVersion 2
+# CHECK-NEXT:       frame-setup PUSH64r killed $r15
+# CHECK-NEXT:       frame-setup SEH_PushReg 126
+# CHECK-NEXT:       frame-setup PUSH64r killed $r14
+# CHECK-NEXT:       frame-setup SEH_PushReg 125
+# CHECK-NEXT:       frame-setup PUSH64r killed $rsi
+# CHECK-NEXT:       frame-setup SEH_PushReg 60
+# CHECK-NEXT:       frame-setup PUSH64r killed $rdi
+# CHECK-NEXT:       frame-setup SEH_PushReg 55
+# CHECK-NEXT:       frame-setup PUSH64r killed $rbx
+# CHECK-NEXT:       frame-setup SEH_PushReg 53
+# CHECK-NEXT:       frame-setup PUSH64r undef $rax
+# CHECK-NEXT:       frame-setup SEH_StackAlloc 8
+# CHECK-NEXT:       frame-setup SEH_EndPrologue
+# CHECK:            SEH_BeginEpilogue
+# CHECK-NEXT:       $rsp = frame-destroy ADD64ri32 $rsp, 8
+# CHECK-NEXT:       SEH_UnwindV2Start
+# CHECK-NEXT:       $rbx = frame-destroy POP64r
+# CHECK-NEXT:       $rdi = frame-destroy POP64r
+# CHECK-NEXT:       $rsi = frame-destroy POP64r
+# CHECK-NEXT:       $r14 = frame-destroy POP64r
+# CHECK-NEXT:       $r15 = frame-destroy POP64r
+# CHECK-NEXT:       SEH_EndEpilogue
+# CHECK-NEXT:       RET64
+
+--- |
+  define void @push_pop_same() {
+    %small_alloca = alloca i32, align 4
+    ret void
+  }
+  
+  define i32 @push_pop_different(i32 %x, i32 %y) {
+    %small_alloca = alloca i32, align 4
+    %sum = add i32 %x, %y
+    ret i32 %sum
+  }
+  
+  define void @push_add(ptr %a, ptr %b, ptr %out) {
+    %small_alloca = alloca i32, align 4
+    %av = load i256, ptr %a, align 16
+    %bv = load i256, ptr %b, align 16
+    %r = mul i256 %av, %bv
+    store i256 %r, ptr %out, align 16
+    ret void
+  }
+  
+  !llvm.module.flags = !{!0}
+  
+  !0 = !{i32 1, !"winx64-eh-unwindv2", i32 2}
+...
+---
+name:            push_pop_same
+body:             |
+  bb.0 (%ir-block.0):
+    frame-setup PUSH64r undef $rax, implicit-def $rsp, implicit $rsp
+    frame-setup SEH_StackAlloc 8
+    frame-setup SEH_EndPrologue
+    SEH_BeginEpilogue
+    $rax = frame-destroy POP64r implicit-def $rsp, implicit $rsp
+    SEH_EndEpilogue
+    RET64
+...
+---
+name:            push_pop_different
+body:             |
+  bb.0 (%ir-block.0):
+    frame-setup PUSH64r undef $rax, implicit-def $rsp, implicit $rsp
+    frame-setup SEH_StackAlloc 8
+    frame-setup SEH_EndPrologue
+    renamable $edx = KILL $edx, implicit-def $rdx
+    renamable $ecx = KILL $ecx, implicit-def $rcx
+    renamable $eax = LEA64_32r killed renamable $rcx, 1, killed renamable $rdx, 0, $noreg
+    SEH_BeginEpilogue
+    $rcx = frame-destroy POP64r implicit-def $rsp, implicit $rsp
+    SEH_EndEpilogue
+    RET64 $eax
+...
+---
+name:            push_add
+body:             |
+  bb.0 (%ir-block.0):
+
+    frame-setup PUSH64r killed $r15, implicit-def $rsp, implicit $rsp
+    frame-setup SEH_PushReg 126
+    frame-setup PUSH64r killed $r14, implicit-def $rsp, implicit $rsp
+    frame-setup SEH_PushReg 125
+    frame-setup PUSH64r killed $rsi, implicit-def $rsp, implicit $rsp
+    frame-setup SEH_PushReg 60
+    frame-setup PUSH64r killed $rdi, implicit-def $rsp, implicit $rsp
+    frame-setup SEH_PushReg 55
+    frame-setup PUSH64r killed $rbx, implicit-def $rsp, implicit $rsp
+    frame-setup SEH_PushReg 53
+    frame-setup PUSH64r undef $rax, implicit-def $rsp, implicit $rsp
+    frame-setup SEH_StackAlloc 8
+    frame-setup SEH_EndPrologue
+    $rsi = MOV64rr $rdx
+    renamable $r9 = MOV64rm renamable $rcx, 1, $noreg, 0, $noreg :: (load (s64) from %ir.a, align 16)
+    renamable $rdi = MOV64rm renamable $rcx, 1, $noreg, 8, $noreg :: (load (s64) from %ir.a + 8, basealign 16)
+    renamable $rbx = MOV64rm renamable $rcx, 1, $noreg, 16, $noreg :: (load (s64) from %ir.a + 16, align 16)
+    renamable $r10 = MOV64rm $rdx, 1, $noreg, 16, $noreg :: (load (s64) from %ir.b + 16, align 16)
+    renamable $r11 = MOV64rm $rdx, 1, $noreg, 0, $noreg :: (load (s64) from %ir.b, align 16)
+    renamable $r14 = MOV64rm $rdx, 1, $noreg, 8, $noreg :: (load (s64) from %ir.b + 8, basealign 16)
+    renamable $r15 = MOV64rm killed renamable $rcx, 1, $noreg, 24, $noreg :: (load (s64) from %ir.a + 24, basealign 16)
+    renamable $r15 = IMUL64rr killed renamable $r15, renamable $r11, implicit-def dead $eflags
+    $rax = MOV64rr $r11
+    MUL64r renamable $rbx, implicit-def $rax, implicit-def $rdx, implicit-def dead $eflags, implicit $rax
+    $rcx = MOV64rr $rax
+    renamable $rbx = IMUL64rr killed renamable $rbx, renamable $r14, implicit-def dead $eflags
+    renamable $rbx = ADD64rr killed renamable $rbx, killed renamable $rdx, implicit-def dead $eflags
+    renamable $rbx = ADD64rr killed renamable $rbx, killed renamable $r15, implicit-def dead $eflags
+    $r15 = MOV64rr $r10
+    renamable $r15 = IMUL64rr killed renamable $r15, renamable $rdi, implicit-def dead $eflags
+    $rax = MOV64rr killed $r10
+    MUL64r renamable $r9, implicit-def $rax, implicit-def $rdx, implicit-def dead $eflags, implicit $rax
+    $r10 = MOV64rr $rax
+    renamable $rdx = ADD64rr killed renamable $rdx, killed renamable $r15, implicit-def dead $eflags
+    renamable $r15 = MOV64rm killed renamable $rsi, 1, $noreg, 24, $noreg :: (load (s64) from %ir.b + 24, basealign 16)
+    renamable $r15 = IMUL64rr killed renamable $r15, renamable $r9, implicit-def dead $eflags
+    renamable $r15 = ADD64rr killed renamable $r15, killed renamable $rdx, implicit-def dead $eflags
+    renamable $r10 = ADD64rr killed renamable $r10, killed renamable $rcx, implicit-def $eflags
+    renamable $r15 = ADC64rr killed renamable $r15, killed renamable $rbx, implicit-def dead $eflags, implicit killed $eflags
+    $rax = MOV64rr $r9
+    MUL64r renamable $r11, implicit-def $rax, implicit-def $rdx, implicit-def dead $eflags, implicit $rax
+    $rcx = MOV64rr $rdx
+    $rsi = MOV64rr $rax
+    $rax = MOV64rr $rdi
+    MUL64r killed renamable $r11, implicit-def $rax, implicit-def $rdx, implicit-def dead $eflags, implicit $rax
+    $r11 = MOV64rr $rdx
+    $rbx = MOV64rr $rax
+    renamable $rbx = ADD64rr killed renamable $rbx, killed renamable $rcx, implicit-def $eflags
+    renamable $r11 = ADC64ri32 killed renamable $r11, 0, implicit-def dead $eflags, implicit killed $eflags
+    $rax = MOV64rr killed $r9
+    MUL64r renamable $r14, implicit-def $rax, implicit-def $rdx, implicit-def dead $eflags, implicit $rax
+    $rcx = MOV64rr $rdx
+    $r9 = MOV64rr $rax
+    renamable $r9 = ADD64rr killed renamable $r9, killed renamable $rbx, implicit-def $eflags
+    renamable $rcx = ADC64rr killed renamable $rcx, killed renamable $r11, implicit-def $eflags, implicit killed $eflags
+    renamable $al = SETCCr 2, implicit killed $eflags
+    renamable $r11d = MOVZX32rr8 killed renamable $al, implicit-def $r11
+    $rax = MOV64rr killed $rdi
+    MUL64r killed renamable $r14, implicit-def $rax, implicit-def $rdx, implicit-def dead $eflags, implicit $rax
+    renamable $rax = ADD64rr killed renamable $rax, killed renamable $rcx, implicit-def $eflags
+    renamable $rdx = ADC64rr killed renamable $rdx, killed renamable $r11, implicit-def dead $eflags, implicit killed $eflags
+    renamable $rax = ADD64rr killed renamable $rax, killed renamable $r10, implicit-def $eflags
+    renamable $rdx = ADC64rr killed renamable $rdx, killed renamable $r15, implicit-def dead $eflags, implicit killed $eflags
+    MOV64mr renamable $r8, 1, $noreg, 0, $noreg, killed renamable $rsi :: (store (s64) into %ir.out, align 16)
+    MOV64mr renamable $r8, 1, $noreg, 8, $noreg, killed renamable $r9 :: (store (s64) into %ir.out + 8, basealign 16)
+    MOV64mr renamable $r8, 1, $noreg, 16, $noreg, killed renamable $rax :: (store (s64) into %ir.out + 16, align 16)
+    MOV64mr killed renamable $r8, 1, $noreg, 24, $noreg, killed renamable $rdx :: (store (s64) into %ir.out + 24, basealign 16)
+    SEH_BeginEpilogue
+    $rsp = frame-destroy ADD64ri32 $rsp, 8, implicit-def dead $eflags
+    $rbx = frame-destroy POP64r implicit-def $rsp, implicit $rsp
+    $rdi = frame-destroy POP64r implicit-def $rsp, implicit $rsp
+    $rsi = frame-destroy POP64r implicit-def $rsp, implicit $rsp
+    $r14 = frame-destroy POP64r implicit-def $rsp, implicit $rsp
+    $r15 = frame-destroy POP64r implicit-def $rsp, implicit $rsp
+    SEH_EndEpilogue
+    RET64
+...