Author: eleviant Date: 2026-07-21T12:44:23+02:00 New Revision: 6d8d3f851820a61054000dbeaed3ab8e212685d3
URL: https://github.com/llvm/llvm-project/commit/6d8d3f851820a61054000dbeaed3ab8e212685d3 DIFF: https://github.com/llvm/llvm-project/commit/6d8d3f851820a61054000dbeaed3ab8e212685d3.diff LOG: [clang] Add flag for making pointer subtraction defined (#196392) The C and C++ standards require both operands of pointer subtraction to refer to elements of the same array object. Clang/LLVM currently relies on this rule in several optimizations: - `inbounds` GEP introduces UB assumptions once the computed address escapes the originating object bounds. - `sdiv exact` assumes %op1 is divisable by %op2 otherwise it is a poison value. The first issue may be addressed with -fwrapv-pointer command line option, however there is no option in clang to mitigate the second issue. Patch adds a new -fdefined-pointer-subtraction to address this. Added: clang/test/CodeGen/ptr-subtract-stable.c Modified: clang/docs/ReleaseNotes.md clang/docs/UsersManual.md clang/include/clang/Basic/LangOptions.def clang/include/clang/Options/Options.td clang/lib/CodeGen/CGExprScalar.cpp clang/lib/Driver/ToolChains/Clang.cpp Removed: ################################################################################ diff --git a/clang/docs/ReleaseNotes.md b/clang/docs/ReleaseNotes.md index 21f0eb1b7cf9f..28e4c98f63bdf 100644 --- a/clang/docs/ReleaseNotes.md +++ b/clang/docs/ReleaseNotes.md @@ -115,6 +115,9 @@ latest release, please see the [Clang Web Site](https://clang.llvm.org) or the ### New Compiler Flags +- New option `-fdefined-pointer-subtraction` added to preserve stable semantics + when subtracting pointers to unrelated objects. + ### Deprecated Compiler Flags ### Modified Compiler Flags diff --git a/clang/docs/UsersManual.md b/clang/docs/UsersManual.md index 9bccf8e175298..54891619f1924 100644 --- a/clang/docs/UsersManual.md +++ b/clang/docs/UsersManual.md @@ -3002,6 +3002,22 @@ are listed below. ``aligned`` attribute, this option is ignored. ``` +```{eval-rst} +.. option:: -fdefined-pointer-subtraction + + The C and C++ standards require both operands of a pointer subtraction to + refer to elements of the same array object. Clang normally exploits this + rule when lowering pointer subtraction operations, for example by emitting + IR constructs such as ``sdiv exact`` that rely on the computed byte offset + being an exact multiple of the pointee size. + + ``-fdefined-pointer-subtraction`` disables these assumptions and emits IR + that preserves the behavior of pointer subtraction even when the standard + requirements are violated. This is primarily intended for low-level code, + such as kernels and boot loaders, that performs pointer arithmetic over + externally defined memory layouts rather than ordinary C or C++ objects. +``` + (strict_aliasing)= (strict-aliasing)= diff --git a/clang/include/clang/Basic/LangOptions.def b/clang/include/clang/Basic/LangOptions.def index 1fb491a54a278..0bccf316f2daf 100644 --- a/clang/include/clang/Basic/LangOptions.def +++ b/clang/include/clang/Basic/LangOptions.def @@ -534,6 +534,8 @@ LANGOPT(EnableLifetimeSafetyTUAnalysis, 1, 0, Benign, "Lifetime safety at transl LANGOPT(PreserveVec3Type, 1, 0, NotCompatible, "Preserve 3-component vector type") LANGOPT(Reflection , 1, 0, NotCompatible, "C++26 Reflection") +LANGOPT(StablePointerSubtraction, 1, 0, NotCompatible, "Make unaligned pointer subtraction stable") + #undef LANGOPT #undef ENUM_LANGOPT #undef VALUE_LANGOPT diff --git a/clang/include/clang/Options/Options.td b/clang/include/clang/Options/Options.td index 4afb089e8a51f..07fec9e9aac21 100644 --- a/clang/include/clang/Options/Options.td +++ b/clang/include/clang/Options/Options.td @@ -4933,6 +4933,19 @@ def fno_wrapv : Flag<["-"], "fno-wrapv">, Group<f_Group>, def fwrapv_pointer : Flag<["-"], "fwrapv-pointer">, Group<f_Group>, Visibility<[ClangOption, CLOption, CC1Option, FlangOption, FC1Option]>, HelpText<"Treat pointer overflow as two's complement">; +def fdefined_pointer_subtraction : + Flag<["-"], "fdefined-pointer-subtraction">, Group<f_Group>, + Visibility<[ClangOption, CC1Option]>, + HelpText< + "Define subtraction of pointers not aligned to object boundaries" + >, + DocBrief< + "When subtracting two pointers, do not assume that the byte diff erence is an " + "exact multiple of the pointee type size, thereby preventing such subtraction " + "from resulting in undefined behavior. Users should prefer casting pointers " + "to ``char *`` before subtracting them rather than relying on this flag." + >, + MarshallingInfoFlag<LangOpts<"StablePointerSubtraction">>; def fno_wrapv_pointer : Flag<["-"], "fno-wrapv-pointer">, Group<f_Group>, Visibility<[ClangOption, CLOption, FlangOption]>; def fwritable_strings : Flag<["-"], "fwritable-strings">, Group<f_Group>, diff --git a/clang/lib/CodeGen/CGExprScalar.cpp b/clang/lib/CodeGen/CGExprScalar.cpp index 81016d3d5d308..8783b43846434 100644 --- a/clang/lib/CodeGen/CGExprScalar.cpp +++ b/clang/lib/CodeGen/CGExprScalar.cpp @@ -5021,6 +5021,8 @@ Value *ScalarExprEmitter::EmitSub(const BinOpInfo &op) { divisor = CGF.CGM.getSize(elementSize); } + if (CGF.getLangOpts().StablePointerSubtraction) + return Builder.CreateSDiv( diff InChars, divisor, "sub.ptr.div"); // Otherwise, do a full sdiv. This uses the "exact" form of sdiv, since // pointer diff erence in C is only defined in the case where both operands // are pointing to elements of an array. diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp index 8c9b98795b194..fb24d0b877ca6 100644 --- a/clang/lib/Driver/ToolChains/Clang.cpp +++ b/clang/lib/Driver/ToolChains/Clang.cpp @@ -5386,6 +5386,9 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA, CmdArgs.push_back("-funified-lto"); } + if (Args.hasArg(options::OPT_fdefined_pointer_subtraction)) + CmdArgs.push_back("-fdefined-pointer-subtraction"); + // If CollectArgsForIntegratedAssembler() isn't called below, claim the args // it claims when not running an assembler. Otherwise, clang would emit // "argument unused" warnings for assembler flags when e.g. adding "-E" to diff --git a/clang/test/CodeGen/ptr-subtract-stable.c b/clang/test/CodeGen/ptr-subtract-stable.c new file mode 100644 index 0000000000000..88581fa51c4b1 --- /dev/null +++ b/clang/test/CodeGen/ptr-subtract-stable.c @@ -0,0 +1,15 @@ +// RUN: %clang -S -emit-llvm -O2 --target=x86_64-windows-msvc -fdefined-pointer-subtraction -fms-extensions %s -o - | FileCheck %s + +// Check that pointer subtraction isn't nuw/nsv and sdiv isn't exact +// CHECK-LABEL: i64 @sub(ptr noundef %p, ptr noundef %q) +// CHECK-NEXT: entry: +// CHECK-NEXT: %sub.ptr.lhs.cast = ptrtoint ptr %p to i64 +// CHECK-NEXT: %sub.ptr.rhs.cast = ptrtoint ptr %q to i64 +// CHECK-NEXT: %sub.ptr.sub = sub i64 %sub.ptr.lhs.cast, %sub.ptr.rhs.cast +// CHECK-NEXT: %sub.ptr.div = sdiv i64 %sub.ptr.sub, 4 +// CHECK-NEXT: ret i64 %sub.ptr.div + +__declspec(noinline) long long sub(long* p, long* q) { + return p - q; +} + _______________________________________________ cfe-commits mailing list [email protected] https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
