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This patch addresses an ABI compatibility issue with empty aggregate

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parameters.  Examples of these are:

  struct { } a;
  union { } b[256];
  int a[0];

An empty aggregate has an address, although dereferencing that address is
pointless.  When passed as a parameter, an empty aggregate does not consume
a protocol register, nor does it consume a doubleword in the parameter save
area.  Passing an empty aggregate by reference passes an address just as
for any other aggregate.  Returning an empty aggregate uses GPR3 as a hidden
address of the return value location, just as for any other aggregate.

The patch modifies PPCTargetLowering::LowerFormalArguments_64SVR4 and
PPCTargetLowering::LowerCall_64SVR4 to properly skip empty aggregate
parameters passed by value.  The handling of return values and by-reference
parameters was already correct.

Built on powerpc64-unknown-linux-gnu and tested with no new regressions.
A test case is included to test proper handling of empty aggregate
parameters on both sides of the function call protocol.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@167090 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Bill Schmidt 2012-10-31 01:15:05 +00:00
parent bab24216cc
commit 42d43351b2
2 changed files with 70 additions and 0 deletions
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19
lib/Target/PowerPC/PPCISelLowering.cpp
View File

@ -2091,6 +2091,19 @@ PPCTargetLowering::LowerFormalArguments_64SVR4(
// ObjSize is the true size, ArgSize rounded up to multiple of registers.
ObjSize = Flags.getByValSize();
ArgSize = ((ObjSize + PtrByteSize - 1)/PtrByteSize) * PtrByteSize;
// Empty aggregate parameters do not take up registers. Examples:
// struct { } a;
// union { } b;
// int c[0];
// etc. However, we have to provide a place-holder in InVals, so
// pretend we have an 8-byte item at the current address for that
// purpose.
if (!ObjSize) {
int FI = MFI->CreateFixedObject(PtrByteSize, ArgOffset, true);
SDValue FIN = DAG.getFrameIndex(FI, PtrVT);
InVals.push_back(FIN);
continue;
}
// All aggregates smaller than 8 bytes must be passed right-justified.
if (ObjSize < PtrByteSize)
CurArgOffset = CurArgOffset + (PtrByteSize - ObjSize);
@ -3655,6 +3668,12 @@ PPCTargetLowering::LowerCall_64SVR4(SDValue Chain, SDValue Callee,
// These are the proper values we need for right-justifying the
// aggregate in a parameter register.
unsigned Size = Flags.getByValSize();
// An empty aggregate parameter takes up no storage and no
// registers.
if (Size == 0)
continue;
// All aggregates smaller than 8 bytes must be passed right-justified.
if (Size==1 || Size==2 || Size==4) {
EVT VT = (Size==1) ? MVT::i8 : ((Size==2) ? MVT::i16 : MVT::i32);

51
test/CodeGen/PowerPC/emptystruct.ll Normal file
View File

@ -0,0 +1,51 @@
; RUN: llc -mcpu=pwr7 -O0 < %s | FileCheck %s
; This tests correct handling of empty aggregate parameters and return values.
; An empty parameter passed by value does not consume a protocol register or
; a parameter save area doubleword. An empty parameter passed by reference
; is treated as any other pointer parameter. An empty aggregate return value
; is treated as any other aggregate return value, passed via address as a
; hidden parameter in GPR3. In this example, GPR3 contains the return value
; address, GPR4 contains the address of e2, and e1 and e3 are not passed or
; received.
target datalayout = "E-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-f128:128:128-v128:128:128-n32:64"
target triple = "powerpc64-unknown-linux-gnu"
%struct.empty = type {}
define void @callee(%struct.empty* noalias sret %agg.result, %struct.empty* byval %a1, %struct.empty* %a2, %struct.empty* byval %a3) nounwind {
entry:
%a2.addr = alloca %struct.empty*, align 8
store %struct.empty* %a2, %struct.empty** %a2.addr, align 8
%0 = load %struct.empty** %a2.addr, align 8
%1 = bitcast %struct.empty* %agg.result to i8*
%2 = bitcast %struct.empty* %0 to i8*
call void @llvm.memcpy.p0i8.p0i8.i64(i8* %1, i8* %2, i64 0, i32 1, i1 false)
ret void
}
; CHECK: callee:
; CHECK: std 4,
; CHECK: std 3,
; CHECK-NOT: std 5,
; CHECK-NOT: std 6,
; CHECK: blr
declare void @llvm.memcpy.p0i8.p0i8.i64(i8* nocapture, i8* nocapture, i64, i32, i1) nounwind
define void @caller(%struct.empty* noalias sret %agg.result) nounwind {
entry:
%e1 = alloca %struct.empty, align 1
%e2 = alloca %struct.empty, align 1
%e3 = alloca %struct.empty, align 1
call void @callee(%struct.empty* sret %agg.result, %struct.empty* byval %e1, %struct.empty* %e2, %struct.empty* byval %e3)
ret void
}
; CHECK: caller:
; CHECK: addi 4,
; CHECK: std 3,
; CHECK-NOT: std 5,
; CHECK-NOT: std 6,
; CHECK: bl callee