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[IPSCCP] Fix a bug that the "returned" attribute is not cleared when function is optimized to return undef 

In IPSCCP when a function is optimized to return undef, it should clear the returned attribute for all its input arguments
and its corresponding call sites.

The bug is exposed when the value of an input argument of the function is assigned to a physical register and
because of the argument having a returned attribute, the value of this physical register will continue to be used
as the function return value right after the call instruction returns, even if the value that this register holds may
be clobbered during the function call. This potentially results in incorrect values being used afterwards.

Reviewed By: jdoerfert, fhahn

Differential Revision: https://reviews.llvm.org/D84220
This commit is contained in:
Congzhe Cao 2020-09-02 11:02:58 -04:00 committed by Danilo C. Grael
parent b6b63684b1
commit ec489ae048
2 changed files with 81 additions and 0 deletions
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19
llvm/lib/Transforms/Scalar/SCCP.cpp
View File

@ -23,6 +23,7 @@
#include "llvm/ADT/MapVector.h"
#include "llvm/ADT/PointerIntPair.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
@ -2112,9 +2113,27 @@ bool llvm::runIPSCCP(
}
// Zap all returns which we've identified as zap to change.
SmallSetVector<Function *, 8> FuncZappedReturn;
for (unsigned i = 0, e = ReturnsToZap.size(); i != e; ++i) {
Function *F = ReturnsToZap[i]->getParent()->getParent();
ReturnsToZap[i]->setOperand(0, UndefValue::get(F->getReturnType()));
// Record all functions that are zapped.
FuncZappedReturn.insert(F);
}
// Remove the returned attribute for zapped functions and the
// corresponding call sites.
for (Function *F : FuncZappedReturn) {
for (Argument &A : F->args())
F->removeParamAttr(A.getArgNo(), Attribute::Returned);
for (Use &U : F->uses()) {
// Skip over blockaddr users.
if (isa<BlockAddress>(U.getUser()))
continue;
CallBase *CB = cast<CallBase>(U.getUser());
for (Use &Arg : CB->args())
CB->removeParamAttr(CB->getArgOperandNo(&Arg), Attribute::Returned);
}
}
// If we inferred constant or undef values for globals variables, we can

62
llvm/test/Transforms/SCCP/ipsccp-clear-returned.ll Normal file
View File

@ -0,0 +1,62 @@
; if IPSCCP determines a function returns undef,
; then the "returned" attribute of input arguments
; should be cleared.
; RUN: opt < %s -ipsccp -S | FileCheck %s
define i32 @main() {
; CHECK-LABEL: @main
entry:
; CHECK-NEXT: entry:
%call = call i32 @func_return_undef(i32 returned 1)
; CHECK: call i32 @func_return_undef(i32 1)
; CHECK-NOT: returned
ret i32 %call
; CHECK: ret i32 1
}
define internal i32 @func_return_undef(i32 returned %arg) {
; CHECK: {{define.*@func_return_undef}}
; CHECK-NOT: returned
entry:
; CHECK-NEXT: entry:
; CHECK-NEXT: {{ret.*undef}}
ret i32 %arg
}
; The only case that users of zapped functions are non-call site
; users is that they are blockaddr users. Skip them because we
; want to remove the returned attribute for call sites
; CHECK: {{define.*@blockaddr_user}}
; CHECK-NOT: returned
define internal i32 @blockaddr_user(i1 %c, i32 returned %d) {
entry:
br i1 %c, label %bb1, label %bb2
bb1:
br label %branch.block
bb2:
br label %branch.block
branch.block:
%addr = phi i8* [blockaddress(@blockaddr_user, %target1), %bb1], [blockaddress(@blockaddr_user, %target2), %bb2]
indirectbr i8* %addr, [label %target1, label %target2]
target1:
br label %target2
; CHECK: ret i32 undef
target2:
ret i32 %d
}
define i32 @call_blockaddr_user(i1 %c) {
; CHECK-LABEL: define i32 @call_blockaddr_user(
; CHECK-NEXT: %r = call i32 @blockaddr_user(i1 %c
; CHECK-NOT: returned
; CHECK-NEXT: ret i32 10
%r = call i32 @blockaddr_user(i1 %c, i32 returned 10)
ret i32 %r
}