[clang] [clang][Driver] Pass --cstdlib through to multilibs (PR #203884)

[Volodymyr Turanskyy via cfe-commits]

https://github.com/voltur01 created https://github.com/llvm/llvm-project/pull/203884

Pass the --cstdlib through to the multilibs backend so that the value specified on the command line can be used inside the multilibs YAML file.

Assisted-by: codex, reviewed and tested by me.

>From e72ca196dfc1a9fa3542cef91ac3cd40698af112 Mon Sep 17 00:00:00 2001
From: Volodymyr Turanskyy <volodymyr.turanskyy at arm.com>
Date: Mon, 15 Jun 2026 13:38:26 +0100
Subject: [PATCH] [clang][Driver] Pass --cstdlib through to multilibs

Pass the --cstdlib through to the multilibs backend so that the value specified on the command line can be used inside the multilibs YAML file.

Assisted-by: codex, reviewed and tested by me.
---
 clang/lib/Driver/ToolChain.cpp                  |  6 ++++++
 clang/test/Driver/baremetal-multilib.yaml       | 11 +++++++++++
 clang/test/Driver/print-multi-selection-flags.c |  4 ++++
 3 files changed, 21 insertions(+)

diff --git a/clang/lib/Driver/ToolChain.cpp b/clang/lib/Driver/ToolChain.cpp
index 8db726cedfe90..a4a4f49036eda 100644
--- a/clang/lib/Driver/ToolChain.cpp
+++ b/clang/lib/Driver/ToolChain.cpp
@@ -492,6 +492,12 @@ ToolChain::getMultilibFlags(const llvm::opt::ArgList &Args) const {
 
   processMultilibCustomFlags(Result, Args);
 
+  if (Arg *CStdLibArg = Args.getLastArg(options::OPT_cstdlib_EQ)) {
+    Result.push_back(std::string(CStdLibArg->getOption().getPrefixedName()) +
+                     CStdLibArg->getValue());
+    CStdLibArg->claim();
+  }
+
   // Include fno-exceptions and fno-rtti
   // to improve multilib selection
   if (getRTTIMode() == ToolChain::RTTIMode::RM_Disabled)
diff --git a/clang/test/Driver/baremetal-multilib.yaml b/clang/test/Driver/baremetal-multilib.yaml
index c2b37fad97dea..17df2972116b1 100644
--- a/clang/test/Driver/baremetal-multilib.yaml
+++ b/clang/test/Driver/baremetal-multilib.yaml
@@ -14,6 +14,14 @@
 # CHECK-SAME: "-lc"
 # CHECK-SAME: "-o" "{{.*}}.tmp.out"
 
+# RUN: %clang --multi-lib-config=%s -no-canonical-prefixes -x c++ %s -### -Werror -o %t.out 2>&1 \
+# RUN:     --target=thumbv8m.main-none-eabihf --cstdlib=picolibc --sysroot= \
+# RUN:   | FileCheck --check-prefix=CHECK-CSTDLIB %s
+# CHECK-CSTDLIB:      "-cc1" "-triple" "thumbv8m.main-unknown-none-eabihf"
+# CHECK-CSTDLIB-SAME: "-internal-isystem" "[[SYSROOT:[^"]*]]/bin/../lib/clang-runtimes/arm-none-eabi/thumb/v8-m.main/fp/picolibc/include/c++/v1"
+# CHECK-CSTDLIB-NEXT: ld{{(.exe)?}}" "-Bstatic"
+# CHECK-CSTDLIB-SAME: "-L[[SYSROOT]]/bin/../lib/clang-runtimes/arm-none-eabi/thumb/v8-m.main/fp/picolibc/lib"
+
 # RUN: %clang --multi-lib-config=%s -no-canonical-prefixes -x c++ %s -### -o %t.out 2>&1 \
 # RUN:     --target=thumbv7em-none-eabi -mfpu=fpv4-sp-d16 --sysroot= \
 # RUN:   | FileCheck --check-prefix=CHECK-NO-MATCH %s
@@ -106,6 +114,9 @@ Variants:
 - Dir: arm-none-eabi/thumb/v8-m.main/fp
   Flags: [--target=thumbv8m.main-unknown-none-eabihf, -mfpu=fpv5-d16]
 
+- Dir: arm-none-eabi/thumb/v8-m.main/fp/picolibc
+  Flags: [--target=thumbv8m.main-unknown-none-eabihf, --cstdlib=picolibc, -mfpu=fpv5-d16]
+
 - Dir: arm-none-eabi/thumb/v8.1-m.main/fp
   Flags: [--target=thumbv8.1m.main-unknown-none-eabihf, -mfpu=fp-armv8-fullfp16-sp-d16]
 
diff --git a/clang/test/Driver/print-multi-selection-flags.c b/clang/test/Driver/print-multi-selection-flags.c
index ba7b325892f9c..8d068fdb48a34 100644
--- a/clang/test/Driver/print-multi-selection-flags.c
+++ b/clang/test/Driver/print-multi-selection-flags.c
@@ -71,6 +71,10 @@
 // RUN: %clang -multi-lib-config=%S/Inputs/multilib/empty.yaml -print-multi-flags-experimental --target=aarch64-none-elf -mbranch-protection=standard | FileCheck --check-prefix=CHECK-BRANCH-PROTECTION %s
 // CHECK-BRANCH-PROTECTION: -mbranch-protection=standard
 
+// RUN: %clang -multi-lib-config=%S/Inputs/multilib/empty.yaml -print-multi-flags-experimental --target=arm-none-eabi --cstdlib=picolibc | FileCheck --check-prefix=CHECK-CSTDLIB %s
+// RUN: %clang -multi-lib-config=%S/Inputs/multilib/empty.yaml -print-multi-flags-experimental --target=arm-none-eabi --cstdlib picolibc | FileCheck --check-prefix=CHECK-CSTDLIB %s
+// CHECK-CSTDLIB: --cstdlib=picolibc
+
 // RUN: %clang -multi-lib-config=%S/Inputs/multilib/empty.yaml -print-multi-flags-experimental --target=arm-none-eabi -mno-unaligned-access | FileCheck --check-prefix=CHECK-NO-UNALIGNED-ACCESS %s
 // RUN: %clang -multi-lib-config=%S/Inputs/multilib/empty.yaml -print-multi-flags-experimental --target=arm-none-eabi -mstrict-align | FileCheck --check-prefix=CHECK-NO-UNALIGNED-ACCESS %s
 // RUN: %clang -multi-lib-config=%S/Inputs/multilib/empty.yaml -print-multi-flags-experimental --target=arm-none-eabi | FileCheck --check-prefix=CHECK-NO-UNALIGNED-ACCESS %s