[AArch64] Reserve a 16 byte aligned amount of fixed stack for win64 varargs

Create a dummy 8 byte fixed object for the unused slot below the first
stored vararg.

Alternative ideas tested but skipped: One could try to align the whole
fixed object to 16, but I haven't found how to add an offset to the stack
frame used in LowerWin64_VASTART.

If only the size of the fixed stack object size is padded but not the offset, via
MFI.CreateFixedObject(alignTo(GPRSaveSize, 16), -(int)GPRSaveSize, false),
PrologEpilogInserter crashes due to "Attempted to reset backwards range!".

This fixes misconceptions about where registers are spilled, since
AArch64FrameLowering.cpp assumes the offset from fixed objects is
aligned to 16 bytes (and the Win64 case there already manually aligns
the offset to 16 bytes).

This fixes cases where local stack allocations could overwrite callee
saved registers on the stack.

Differential Revision: https://reviews.llvm.org/D35720

llvm-svn: 308950

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

@@ -2889,9 +2889,12 @@ void AArch64TargetLowering::saveVarArgRegisters(CCState &CCInfo,
   unsigned GPRSaveSize = 8 * (NumGPRArgRegs - FirstVariadicGPR);
   int GPRIdx = 0;
   if (GPRSaveSize != 0) {
-    if (IsWin64)
+    if (IsWin64) {
       GPRIdx = MFI.CreateFixedObject(GPRSaveSize, -(int)GPRSaveSize, false);
-    else
+      if (GPRSaveSize & 15)
+        // The extra size here, if triggered, will always be 8.
+        MFI.CreateFixedObject(16 - (GPRSaveSize & 15), -(int)alignTo(GPRSaveSize, 16), false);
+    } else
       GPRIdx = MFI.CreateStackObject(GPRSaveSize, 8, false);
 
     SDValue FIN = DAG.getFrameIndex(GPRIdx, PtrVT);

llvm/test/CodeGen/AArch64/aarch64_win64cc_vararg.ll

@@ -59,10 +59,12 @@ entry:
 }
 
 ; CHECK-LABEL: f7:
-; CHECK: sub     sp, sp, #16
+; CHECK: sub     sp, sp, #32
-; CHECK: add     x8, sp, #8
+; CHECK: add     x8, sp, #24
-; CHECK: add     x0, sp, #8
+; CHECK: str     x7, [sp, #24]
-; CHECK: stp     x8, x7, [sp], #16
+; CHECK: add     x0, sp, #24
+; CHECK: str     x8, [sp, #8]
+; CHECK: add     sp, sp, #32
 ; CHECK: ret
 define win64cc i8* @f7(i64 %a0, i64 %a1, i64 %a2, i64 %a3, i64 %a4, i64 %a5, i64 %a6, ...) nounwind {
 entry:

llvm/test/CodeGen/AArch64/win64_vararg.ll

@@ -59,10 +59,12 @@ entry:
 }
 
 ; CHECK-LABEL: f7:
-; CHECK: sub     sp, sp, #16
+; CHECK: sub     sp, sp, #32
-; CHECK: add     x8, sp, #8
+; CHECK: add     x8, sp, #24
-; CHECK: add     x0, sp, #8
+; CHECK: str     x7, [sp, #24]
-; CHECK: stp     x8, x7, [sp], #16
+; CHECK: add     x0, sp, #24
+; CHECK: str     x8, [sp, #8]
+; CHECK: add     sp, sp, #32
 ; CHECK: ret
 define i8* @f7(i64 %a0, i64 %a1, i64 %a2, i64 %a3, i64 %a4, i64 %a5, i64 %a6, ...) nounwind {
 entry:
@@ -79,9 +81,8 @@ entry:
 ; CHECK: stp     x6, x7, [sp, #64]
 ; CHECK: stp     x4, x5, [sp, #48]
 ; CHECK: stp     x2, x3, [sp, #32]
-; CHECK: stp     x8, x1, [sp, #16]
+; CHECK: str     x1, [sp, #24]
-; CHECK: str     x8, [sp, #8]
+; CHECK: stp     x8, x8, [sp], #80
-; CHECK: add     sp, sp, #80
 ; CHECK: ret
 define void @copy1(i64 %a0, ...) nounwind {
 entry:
@@ -93,3 +94,54 @@ entry:
   call void @llvm.va_copy(i8* %cp1, i8* %ap1)
   ret void
 }
+
+declare void @llvm.va_end(i8*)
+declare void @llvm.lifetime.start.p0i8(i64, i8* nocapture) #1
+declare void @llvm.lifetime.end.p0i8(i64, i8* nocapture) #1
+
+declare i32 @__stdio_common_vsprintf(i64, i8*, i64, i8*, i8*, i8*) local_unnamed_addr #3
+declare i64* @__local_stdio_printf_options() local_unnamed_addr #4
+
+; CHECK-LABEL: snprintf
+; CHECK: sub     sp,  sp, #96
+; CHECK: stp     x21, x20, [sp, #16]
+; CHECK: stp     x19, x30, [sp, #32]
+; CHECK: add     x8, sp, #56
+; CHECK: mov     x19, x2
+; CHECK: mov     x20, x1
+; CHECK: mov     x21, x0
+; CHECK: stp     x6, x7, [sp, #80]
+; CHECK: stp     x4, x5, [sp, #64]
+; CHECK: str     x3, [sp, #56]
+; CHECK: str     x8, [sp, #8]
+; CHECK: bl      __local_stdio_printf_options
+; CHECK: ldr     x8, [x0]
+; CHECK: add     x5, sp, #56
+; CHECK: mov     x1, x21
+; CHECK: mov     x2, x20
+; CHECK: orr     x0, x8, #0x2
+; CHECK: mov     x3, x19
+; CHECK: mov     x4, xzr
+; CHECK: bl      __stdio_common_vsprintf
+; CHECK: ldp     x19, x30, [sp, #32]
+; CHECK: ldp     x21, x20, [sp, #16]
+; CHECK: cmp     w0, #0
+; CHECK: csinv   w0, w0, wzr, ge
+; CHECK: add     sp, sp, #96
+; CHECK: ret
+define i32 @snprintf(i8*, i64, i8*, ...) local_unnamed_addr #5 {
+  %4 = alloca i8*, align 8
+  %5 = bitcast i8** %4 to i8*
+  call void @llvm.lifetime.start.p0i8(i64 8, i8* nonnull %5) #2
+  call void @llvm.va_start(i8* nonnull %5)
+  %6 = load i8*, i8** %4, align 8
+  %7 = call i64* @__local_stdio_printf_options() #2
+  %8 = load i64, i64* %7, align 8
+  %9 = or i64 %8, 2
+  %10 = call i32 @__stdio_common_vsprintf(i64 %9, i8* %0, i64 %1, i8* %2, i8* null, i8* %6) #2
+  %11 = icmp sgt i32 %10, -1
+  %12 = select i1 %11, i32 %10, i32 -1
+  call void @llvm.va_end(i8* nonnull %5)
+  call void @llvm.lifetime.end.p0i8(i64 8, i8* nonnull %5) #2
+  ret i32 %12
+}