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Mar 19, 2024
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Add support for sysroot-relative system header search paths #82084

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43 changes: 28 additions & 15 deletions clang/lib/Frontend/CompilerInvocation.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -3191,6 +3191,22 @@ static bool ParseHeaderSearchArgs(HeaderSearchOptions &Opts, ArgList &Args,
bool IsIndexHeaderMap = false;
bool IsSysrootSpecified =
Args.hasArg(OPT__sysroot_EQ) || Args.hasArg(OPT_isysroot);

// Expand a leading `=` to the sysroot if one was passed (and it's not a
// framework flag).
auto PrefixHeaderPath = [IsSysrootSpecified,
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@fanghuaqi fanghuaqi Sep 29, 2025 •
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Hello @etcwilde @jansvoboda11 , I have a question about why IsSysrootSpecified is required, since the sysroot could use default sysroot defined in toolchain. For gcc, it is not required.

If dir begins with ‘=’ or $SYSROOT, then the ‘=’ or $SYSROOT is replaced by the sysroot prefix; see --sysroot and -isysroot.

So it will replace sysroot defined in toolchain, as tested below:

  • gcc -isystem=/include/libncrt: xxxxx/gcc/bin/../riscv64-unknown-elf/include/libncrt, this is expected
  • clang -isystem=/include/libncrt : ignoring nonexistent directory "=/include/libncrt"

Could this feature be added in llvm to match what is done in gcc's behavior?

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Hey! Do you have any feedback regarding the above question? Thanks a ton!

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My understanding is that with this patch, whenever is sysroot specified we match the GCC behavior (assuming the = spelling). Are you specifically asking to add a fallback to <clang>/../<target>/include?

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@fanghuaqi fanghuaqi Oct 11, 2025 •
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My understanding is that with this patch, whenever is sysroot specified we match the GCC behavior (assuming the = spelling). Are you specifically asking to add a fallback to <clang>/../<target>/include?

Thanks @jansvoboda11 for your reply.

I found my issue reported is caused by this

llvm-project/clang/lib/Driver/ToolChains/Clang.cpp

Lines 1084 to 1092 in 53c785d

// If we have a --sysroot, and don't have an explicit -isysroot flag, add an
// -isysroot to the CC1 invocation.
StringRef sysroot = C.getSysRoot();
if (sysroot != "") {
if (!Args.hasArg(options::OPT_isysroot)) {
CmdArgs.push_back("-isysroot");
CmdArgs.push_back(C.getArgs().MakeArgString(sysroot));
}
}

The sysroot returned is empty, but actually it can be computed like this getToolChain().computeSysRoot(), for my case, it is done in

llvm-project/clang/lib/Driver/ToolChains/BareMetal.cpp

Line 165 in 53c785d

std::string BareMetal::computeSysRoot() const {

And locally I modified the code clang/lib/Driver/ToolChains/Clang.cpp in my side like this, then it fixed my issue.

   // If we have a --sysroot, and don't have an explicit -isysroot flag, add an
   // -isysroot to the CC1 invocation.
-  StringRef sysroot = C.getSysRoot();
-  if (sysroot != "") {
-    if (!Args.hasArg(options::OPT_isysroot)) {
-      CmdArgs.push_back("-isysroot");
-      CmdArgs.push_back(C.getArgs().MakeArgString(sysroot));
-    }
+  std::string sysroot = std::string(C.getSysRoot());
+  if (sysroot.empty()) {
+    sysroot = getToolChain().computeSysRoot();
+  }
+  if (!Args.hasArg(options::OPT_isysroot)) {
+    CmdArgs.push_back("-isysroot");
+    CmdArgs.push_back(C.getArgs().MakeArgString(sysroot));
   }

Regarding Are you specifically asking to add a fallback to /..//include?

I think it will be good, since the = is not handled correctly, it directly returned invalid path =/include/libncrt not a sysroot path + /include/libncrt

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I see, thanks for explaining. Do you mind creating a PR? That seems like a reasonable path forward.

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Okay, I will try to create a PR later.

&Opts](const llvm::opt::Arg *A,
bool IsFramework = false) -> std::string {
assert(A->getNumValues() && "Unexpected empty search path flag!");
if (IsSysrootSpecified && !IsFramework && A->getValue()[0] == '=') {
SmallString<32> Buffer;
llvm::sys::path::append(Buffer, Opts.Sysroot,
llvm::StringRef(A->getValue()).substr(1));
return std::string(Buffer);
}
return A->getValue();
};

for (const auto *A : Args.filtered(OPT_I, OPT_F, OPT_index_header_map)) {
if (A->getOption().matches(OPT_index_header_map)) {
// -index-header-map applies to the next -I or -F.
Expand All @@ -3202,16 +3218,7 @@ static bool ParseHeaderSearchArgs(HeaderSearchOptions &Opts, ArgList &Args,
IsIndexHeaderMap ? frontend::IndexHeaderMap : frontend::Angled;

bool IsFramework = A->getOption().matches(OPT_F);
std::string Path = A->getValue();

if (IsSysrootSpecified && !IsFramework && A->getValue()[0] == '=') {
SmallString<32> Buffer;
llvm::sys::path::append(Buffer, Opts.Sysroot,
llvm::StringRef(A->getValue()).substr(1));
Path = std::string(Buffer);
}

Opts.AddPath(Path, Group, IsFramework,
Opts.AddPath(PrefixHeaderPath(A, IsFramework), Group, IsFramework,
/*IgnoreSysroot*/ true);
IsIndexHeaderMap = false;
}
Expand All @@ -3229,12 +3236,18 @@ static bool ParseHeaderSearchArgs(HeaderSearchOptions &Opts, ArgList &Args,
}

for (const auto *A : Args.filtered(OPT_idirafter))
Opts.AddPath(A->getValue(), frontend::After, false, true);
Opts.AddPath(PrefixHeaderPath(A), frontend::After, false, true);
for (const auto *A : Args.filtered(OPT_iquote))
Opts.AddPath(A->getValue(), frontend::Quoted, false, true);
for (const auto *A : Args.filtered(OPT_isystem, OPT_iwithsysroot))
Opts.AddPath(A->getValue(), frontend::System, false,
!A->getOption().matches(OPT_iwithsysroot));
Opts.AddPath(PrefixHeaderPath(A), frontend::Quoted, false, true);

for (const auto *A : Args.filtered(OPT_isystem, OPT_iwithsysroot)) {
if (A->getOption().matches(OPT_iwithsysroot)) {
Opts.AddPath(A->getValue(), frontend::System, false,
/*IgnoreSysRoot=*/false);
continue;
}
Opts.AddPath(PrefixHeaderPath(A), frontend::System, false, true);
}
for (const auto *A : Args.filtered(OPT_iframework))
Opts.AddPath(A->getValue(), frontend::System, true, true);
for (const auto *A : Args.filtered(OPT_iframeworkwithsysroot))
Expand Down
25 changes: 25 additions & 0 deletions clang/test/Preprocessor/sysroot-prefix.c
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Original file line number Diff line number Diff line change
Expand Up @@ -4,6 +4,16 @@
// RUN: %clang_cc1 -v -isysroot /var/empty -I =null -E %s -o /dev/null 2>&1 | FileCheck -check-prefix CHECK-ISYSROOT_SYSROOT_NULL %s
// RUN: %clang_cc1 -v -isysroot /var/empty -isysroot /var/empty/root -I =null -E %s -o /dev/null 2>&1 | FileCheck -check-prefix CHECK-ISYSROOT_ISYSROOT_SYSROOT_NULL %s
// RUN: %clang_cc1 -v -isysroot /var/empty/root -isysroot /var/empty -I =null -E %s -o /dev/null 2>&1 | FileCheck -check-prefix CHECK-ISYSROOT_ISYSROOT_SWAPPED_SYSROOT_NULL %s
// RUN: %clang_cc1 -v -isystem=/usr/include -isysroot /var/empty -E %s -o /dev/null 2>&1 | FileCheck -check-prefix CHECK-ISYSROOT_ISYSTEM_SYSROOT %s
// RUN: %clang_cc1 -v -isystem=/usr/include -E %s -o /dev/null 2>&1 | FileCheck -check-prefix CHECK-ISYSROOT_ISYSTEM_NO_SYSROOT %s
// RUN: %clang_cc1 -v -iquote=/usr/include -isysroot /var/empty -E %s -o /dev/null 2>&1 | FileCheck -check-prefix CHECK-ISYSROOT_IQUOTE_SYSROOT %s
// RUN: %clang_cc1 -v -iquote=/usr/include -E %s -o /dev/null 2>&1 | FileCheck -check-prefix CHECK-ISYSROOT_IQUOTE_NO_SYSROOT %s
// RUN: %clang_cc1 -v -idirafter=/usr/include -isysroot /var/empty -E %s -o /dev/null 2>&1 | FileCheck -check-prefix CHECK-ISYSROOT_IDIRAFTER_SYSROOT %s
// RUN: %clang_cc1 -v -idirafter=/usr/include -E %s -o /dev/null 2>&1 | FileCheck -check-prefix CHECK-ISYSROOT_IDIRAFTER_NO_SYSROOT %s
// RUN: %clang_cc1 -v -isysroot /var/empty -isystem=/usr/include -iwithsysroot /opt/include -isystem=/usr/local/include -E %s -o /dev/null 2>&1 | FileCheck -check-prefix CHECK-INTERLEAVE_I_PATHS %s

// RUN: not %clang_cc1 -v -isysroot /var/empty -E %s -o /dev/null -I 2>&1 | FileCheck -check-prefix CHECK-EMPTY_I_PATH %s
// RUN: %clang_cc1 -v -isysroot /var/empty/usr -E %s -o /dev/null -I= 2>&1 | FileCheck -check-prefix CHECK-SYSROOT_I_PATH %s

// CHECK-ISYSROOT_NO_SYSROOT: ignoring nonexistent directory "/var/empty/include"
// CHECK-ISYSROOT_NO_SYSROOT-NOT: ignoring nonexistent directory "/var/empty/var/empty/include"
Expand All @@ -23,3 +33,18 @@
// CHECK-ISYSROOT_ISYSROOT_SWAPPED_SYSROOT_NULL: ignoring nonexistent directory "/var/empty{{.}}null"
// CHECK-ISYSROOT_ISYSROOT_SWAPPED_SYSROOT_NULL-NOT: ignoring nonexistent directory "=null"

// CHECK-ISYSROOT_ISYSTEM_SYSROOT: ignoring nonexistent directory "/var/empty/usr/include"
// CHECK-ISYSROOT_ISYSTEM_NO_SYSROOT: ignoring nonexistent directory "=/usr/include"

// CHECK-ISYSROOT_IQUOTE_SYSROOT: ignoring nonexistent directory "/var/empty/usr/include"
// CHECK-ISYSROOT_IQUOTE_NO_SYSROOT: ignoring nonexistent directory "=/usr/include"

// CHECK-ISYSROOT_IDIRAFTER_SYSROOT: ignoring nonexistent directory "/var/empty/usr/include"
// CHECK-ISYSROOT_IDIRAFTER_NO_SYSROOT: ignoring nonexistent directory "=/usr/include"

// CHECK-INTERLEAVE_I_PATHS: ignoring nonexistent directory "/var/empty/usr/include"
// CHECK-INTERLEAVE_I_PATHS: ignoring nonexistent directory "/var/empty/opt/include"
// CHECK-INTERLEAVE_I_PATHS: ignoring nonexistent directory "/var/empty/usr/local/include"

// CHECK-EMPTY_I_PATH: argument to '-I' is missing
// CHECK-SYSROOT_I_PATH: ignoring nonexistent directory "/var/empty/usr{{/|\\}}"