maxjhandsome
/
llvm-project
forked from OSchip/llvm-project
1
0
Fork 0
Code Issues Pull Requests Packages Releases Wiki Activity

constexpr: check for overflow in pointer subtraction. 

This is a mess. According to the C++11 standard, pointer subtraction only has
undefined behavior if the difference of the array indices does not fit into a
ptrdiff_t.

However, common implementations effectively perform a char* subtraction first,
and then divide the result by the element size, which can cause overflows in
some cases. Those cases are not considered to be undefined behavior by this
change; perhaps they should be.

llvm-svn: 149490
This commit is contained in:
Richard Smith 2012-02-01 08:10:20 +00:00
parent 6b2bd93918
commit 1b470417e4
2 changed files with 32 additions and 7 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

29
clang/lib/AST/ExprConstant.cpp
View File

@ -4347,6 +4347,9 @@ bool IntExprEvaluator::VisitBinaryOperator(const BinaryOperator *E) {
// - Pointer subtractions must be on elements of the same array.
// - Pointer comparisons must be between members with the same access.
const CharUnits &LHSOffset = LHSValue.getLValueOffset();
const CharUnits &RHSOffset = RHSValue.getLValueOffset();
if (E->getOpcode() == BO_Sub) {
QualType Type = E->getLHS()->getType();
QualType ElementType = Type->getAs<PointerType>()->getPointeeType();
@ -4355,13 +4358,27 @@ bool IntExprEvaluator::VisitBinaryOperator(const BinaryOperator *E) {
if (!HandleSizeof(Info, ElementType, ElementSize))
return false;
CharUnits Diff = LHSValue.getLValueOffset() -
RHSValue.getLValueOffset();
return Success(Diff / ElementSize, E);
}
// FIXME: LLVM and GCC both compute LHSOffset - RHSOffset at runtime,
// and produce incorrect results when it overflows. Such behavior
// appears to be non-conforming, but is common, so perhaps we should
// assume the standard intended for such cases to be undefined behavior
// and check for them.
const CharUnits &LHSOffset = LHSValue.getLValueOffset();
const CharUnits &RHSOffset = RHSValue.getLValueOffset();
// Compute (LHSOffset - RHSOffset) / Size carefully, checking for
// overflow in the final conversion to ptrdiff_t.
APSInt LHS(
llvm::APInt(65, (int64_t)LHSOffset.getQuantity(), true), false);
APSInt RHS(
llvm::APInt(65, (int64_t)RHSOffset.getQuantity(), true), false);
APSInt ElemSize(
llvm::APInt(65, (int64_t)ElementSize.getQuantity(), true), false);
APSInt TrueResult = (LHS - RHS) / ElemSize;
APSInt Result = TrueResult.trunc(Info.Ctx.getIntWidth(E->getType()));
if (Result.extend(65) != TrueResult)
HandleOverflow(Info, E, TrueResult, E->getType());
return Success(Result, E);
}
// C++11 [expr.rel]p3:
// Pointers to void (after pointer conversions) can be compared, with a

10
clang/test/CXX/expr/expr.const/p2-0x.cpp
View File

@ -1,4 +1,4 @@
// RUN: %clang_cc1 -fsyntax-only -std=c++11 -pedantic -verify -fcxx-exceptions %s -fconstexpr-depth 128
// RUN: %clang_cc1 -fsyntax-only -std=c++11 -pedantic -verify -fcxx-exceptions %s -fconstexpr-depth 128 -triple i686-pc-linux-gnu
// A conditional-expression is a core constant expression unless it involves one
// of the following as a potentially evaluated subexpression [...]:
@ -222,6 +222,14 @@ namespace UndefinedBehavior {
constexpr long long ll4 = ll3 - 1; // expected-error {{constant}} expected-note {{ -9223372036854775809 }}
constexpr long long ll5 = ll3 * ll3; // expected-error {{constant}} expected-note {{ 85070591730234615865843651857942052864 }}
// Yikes.
char melchizedek[2200000000];
typedef decltype(melchizedek[1] - melchizedek[0]) ptrdiff_t;
constexpr ptrdiff_t d1 = &melchizedek[0x7fffffff] - &melchizedek[0]; // ok
constexpr ptrdiff_t d2 = &melchizedek[0x80000000u] - &melchizedek[0]; // expected-error {{constant expression}} expected-note {{ 2147483648 }}
constexpr ptrdiff_t d3 = &melchizedek[0] - &melchizedek[0x80000000u]; // ok
constexpr ptrdiff_t d4 = &melchizedek[0] - &melchizedek[0x80000001u]; // expected-error {{constant expression}} expected-note {{ -2147483649 }}
// Unsigned int overflow.
static_assert(65536u * 65536u == 0u, ""); // ok
static_assert(4294967295u + 1u == 0u, ""); // ok