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[GlobalDCE] In VFE support for relative pointers, allow GEP references to the base symbol 

This is for Swift VFE support. In some vtable forms that Swift emits, the "base" of a relative pointer is not the global symbol itself directly, but a GEP into it -- so the pointer is relative to a particular field in the global. So getPointerAtOffset() needs to be able to see through the GEP and allow it in a SUB expression, to correctly recognize the offset as a vtable slot.

Differential Revision: https://reviews.llvm.org/D109169
This commit is contained in:
Kuba Mracek 2021-09-13 14:44:49 -07:00
parent b2032f18c9
commit e80ee4cbd9
2 changed files with 52 additions and 2 deletions
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13
llvm/lib/Analysis/TypeMetadataUtils.cpp
View File

@ -173,10 +173,19 @@ Constant *llvm::getPointerAtOffset(Constant *I, uint64_t Offset, Module &M,
case Instruction::Sub: {
auto *Operand0 = cast<Constant>(C->getOperand(0));
auto *Operand1 = cast<Constant>(C->getOperand(1));
auto *Operand1TargetGlobal = getPointerAtOffset(Operand1, 0, M);
auto StripGEP = [](Constant *C) {
auto *CE = dyn_cast<ConstantExpr>(C);
if (!CE)
return C;
if (CE->getOpcode() != Instruction::GetElementPtr)
return C;
return CE->getOperand(0);
};
auto *Operand1TargetGlobal = StripGEP(getPointerAtOffset(Operand1, 0, M));
// Check that in the "sub (@a, @b)" expression, @b points back to the top
// level global that we're processing. Otherwise bail.
// level global (or a GEP thereof) that we're processing. Otherwise bail.
if (Operand1TargetGlobal != TopLevelGlobal)
return nullptr;

41
llvm/test/Transforms/GlobalDCE/virtual-functions-relative-pointers-gep.ll Normal file
View File

@ -0,0 +1,41 @@
; RUN: opt < %s -passes='globaldce' -S | FileCheck %s
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
declare { i8*, i1 } @llvm.type.checked.load(i8*, i32, metadata)
; A vtable with "relative pointers", slots don't contain pointers to implementations, but instead have an i32 offset from the vtable itself to the implementation.
@vtable = internal unnamed_addr constant { [4 x i32] } { [4 x i32] [
i32 42,
i32 1337,
i32 trunc (i64 sub (i64 ptrtoint (void ()* @vfunc1_live to i64), i64 ptrtoint (i32* getelementptr inbounds ({ [4 x i32] }, { [4 x i32] }* @vtable, i32 0, i32 0, i32 2) to i64)) to i32),
i32 trunc (i64 sub (i64 ptrtoint (void ()* @vfunc2_dead to i64), i64 ptrtoint (i32* getelementptr inbounds ({ [4 x i32] }, { [4 x i32] }* @vtable, i32 0, i32 0, i32 2) to i64)) to i32)
]}, align 8, !type !0, !type !1, !vcall_visibility !{i64 2}
!0 = !{i64 8, !"vfunc1.type"}
!1 = !{i64 12, !"vfunc2.type"}
; CHECK: @vtable = internal unnamed_addr constant { [4 x i32] } { [4 x i32] [
; CHECK-SAME: i32 trunc (i64 sub (i64 ptrtoint (void ()* @vfunc1_live to i64), i64 ptrtoint (i32* getelementptr inbounds ({ [4 x i32] }, { [4 x i32] }* @vtable, i32 0, i32 0, i32 2) to i64)) to i32),
; CHECK-SAME: i32 trunc (i64 sub (i64 0, i64 ptrtoint (i32* getelementptr inbounds ({ [4 x i32] }, { [4 x i32] }* @vtable, i32 0, i32 0, i32 2) to i64)) to i32)
; CHECK-SAME: ] }, align 8, !type !0, !type !1, !vcall_visibility !2
; (1) vfunc1_live is referenced from @main, stays alive
define internal void @vfunc1_live() {
; CHECK: define internal void @vfunc1_live(
ret void
}
; (2) vfunc2_dead is never referenced, gets removed and vtable slot is null'd
define internal void @vfunc2_dead() {
; CHECK-NOT: define internal void @vfunc2_dead(
ret void
}
define void @main() {
%1 = ptrtoint { [4 x i32] }* @vtable to i64 ; to keep @vtable alive
%2 = tail call { i8*, i1 } @llvm.type.checked.load(i8* null, i32 0, metadata !"vfunc1.type")
ret void
}
!999 = !{i32 1, !"Virtual Function Elim", i32 1}
!llvm.module.flags = !{!999}