Warn under -Wlinker-script when multiple entry points are specified

Add EntryCmdWarn.test covering rules and warning behaviour.

Document entry point precedence and the new warning in linker_script.rst,
linker_faq.rst, and diagnostic_reports.rst.

Resolves #1538

Signed-off-by: Rachit Mehta <rachmeht@qti.qualcomm.com>

Author: rachitmeht

docs/userguide/documentation/diagnostic_reports.rst

@@ -85,7 +85,7 @@ General Warning Flags
    * - ``-Werror``
      - Treats all warnings as errors, halting the link process.
    * - ``-Wlinker-script``
-     - Enables warnings specific to linker script issues, such as malformed directives, deprecated syntax, or unsupported constructs.
+     - Enables warnings specific to linker script issues, such as malformed directives, deprecated syntax, unsupported constructs or specifying multiple entry points.
    * - ``-Wlinker-script-memory``
      - Focuses on memory region definitions in linker scripts, such as overlaps or undefined regions.
    * - ``-Wwhole-archive``

docs/userguide/documentation/linker_faq.rst

@@ -344,6 +344,7 @@ Multiple ways to provide entry point to linker
 * Initialising the value of linker symbol "start"
 * Specifying the start address for the first input section in linker script
   (eg: .text : AT(0))
+* When multiple -e options are given, the last one takes effect.
 
 How to obtain a non-executable stack
 --------------------------------------

docs/userguide/documentation/linker_script.rst

@@ -474,6 +474,8 @@ Syntax :- ``ENTRY(symbol)``
   point.
 - The entry point is the first instruction that is executed after a
   program is loaded.
+- When multiple ENTRY() commands appear (in one script or across
+  several -T scripts), the last one takes effect.
 - This command is equivalent to the linker command-line option
   :option:`-e`.
 

include/eld/Diagnostics/DiagLDScript.inc

@@ -39,6 +39,8 @@ DIAG(assert_failed, DiagnosticEngine::Error, "Assertion failed %0")
 DIAG(error_printcmd, DiagnosticEngine::Error, "%0: PRINT: %1")
 DIAG(linker_script_uses_phdrs_no_sections, DiagnosticEngine::Error,
      "Linker Script is using PHDR's but not using SECTIONS command")
+DIAG(warn_multiple_entry, DiagnosticEngine::Warning,
+     "multiple entry points specified; the last one takes effect")
 DIAG(cannot_set_at_address, DiagnosticEngine::Error,
      "Address for section %0 specified with AT cannot be set, as there is a "
      "dependency issue, check the map file for more analysis")

lib/LinkerWrapper/GnuLdDriver.cpp

@@ -719,7 +719,7 @@ bool GnuLdDriver::processOptions(llvm::opt::InputArgList &Args) {
       T::fatal_internal_errors, T::no_fatal_internal_errors, /*default=*/false);
   Config.options().setFatalInternalErrors(enableFatalInternalErrors);
 
-  // set up entry point from -e
+  // set up entry point from -e;
   if (llvm::opt::Arg *arg = Args.getLastArg(T::entrypoint)) {
     Config.options().setEntry(arg->getValue());
     Config.options().setEntryFromCmdLine();

lib/Script/EntryCmd.cpp

@@ -33,7 +33,11 @@ void EntryCmd::dump(llvm::raw_ostream &Outs) const {
 
 eld::Expected<void> EntryCmd::activate(Module &CurModule) {
   GeneralOptions &Options = CurModule.getConfig().options();
-  if (!EntrySymbol.empty() && !Options.isEntryFromCmdLine())
-    Options.setEntry(EntrySymbol);
+  if (!EntrySymbol.empty()) {
+    if (Options.hasEntry() && CurModule.getConfig().showLinkerScriptWarnings())
+      CurModule.getConfig().raise(Diag::warn_multiple_entry);
+    if (!Options.isEntryFromCmdLine())
+      Options.setEntry(EntrySymbol);
+  }
   return eld::Expected<void>();
 }

test/Common/standalone/linkerscript/EntryCmd/EntryCmdWarn.test

@@ -0,0 +1,53 @@
+#---EntryCmdWarn.test-------------------- Executable,LS ----------------------#
+#BEGIN_COMMENT
+# Tests that -Wlinker-script emits a warning when multiple entry points are
+# specified, and that the warning is suppressed without -Wlinker-script.
+#
+# Warning cases:
+#   Case 1: Two ENTRY() in one script  -- warns once with -Wlinker-script.
+#   Case 2: -e combined with a single script ENTRY() -- warns with -Wlinker-script.
+#   Case 3: Multiple -e on command line -- warns once with -Wlinker-script.
+#   Case 4: Cross-script two ENTRY()   -- warns with -Wlinker-script.
+#   Case 5: Single ENTRY(), no -e      -- no warning even with -Wlinker-script.
+#   Case 6: Two ENTRY() without -Wlinker-script -- no warning.
+#   Case 7: Three ENTRY() in one script -- warns once per overriding ENTRY().
+#END_COMMENT
+#START_TEST
+RUN: %clang %clangopts -c %p/Inputs/1.c -o %t1.o -ffunction-sections
+
+# Case 1: Two ENTRY() in one script -- exactly one warning.
+RUN: %link %linkopts %t1.o -T %p/Inputs/two_entries.t -Wlinker-script -o %t.warn1.out 2>&1 | \
+RUN:   %filecheck %s --check-prefix=WARN1
+#WARN1-COUNT-1: Warning: multiple entry points specified; the last one takes effect
+#WARN1-NOT: Warning: multiple entry points specified; the last one takes effect
+
+# Case 2: -e combined with a single script ENTRY() -- warn.
+RUN: %link %linkopts %t1.o -T %p/Inputs/script.t -e bar -Wlinker-script -o %t.warn2.out 2>&1 | \
+RUN:   %filecheck %s --check-prefix=WARN
+#WARN: Warning: multiple entry points specified; the last one takes effect
+
+# Case 3: Multiple -e on command line -- exactly one warning.
+RUN: %link %linkopts %t1.o -T %p/Inputs/script.t -e foo -e bar -Wlinker-script -o %t.warn3.out 2>&1 | \
+RUN:   %filecheck %s --check-prefix=WARN3
+#WARN3-COUNT-1: Warning: multiple entry points specified; the last one takes effect
+#WARN3-NOT: Warning: multiple entry points specified; the last one takes effect
+
+# Case 4: Two ENTRY() across two scripts -- warn.
+RUN: %link %linkopts %t1.o -T %p/Inputs/entry_bar.t -T %p/Inputs/entry_foo.t -Wlinker-script -o %t.warn4.out 2>&1 | \
+RUN:   %filecheck %s --check-prefix=WARN
+
+# Case 5: Single ENTRY(), no -e -- no warning even with -Wlinker-script.
+RUN: %link %linkopts %t1.o -T %p/Inputs/script.t -Wlinker-script -o %t.nowarn1.out 2>&1 | \
+RUN:   %filecheck %s --allow-empty --check-prefix=NOWARN
+#NOWARN-NOT: multiple entry points
+
+# Case 6: Two ENTRY() without -Wlinker-script -- no warning.
+RUN: %link %linkopts %t1.o -T %p/Inputs/two_entries.t -o %t.nowarn2.out 2>&1 | \
+RUN:   %filecheck %s --allow-empty --check-prefix=NOWARN
+
+# Case 7: Three ENTRY() in one script -- one warning per overriding ENTRY() (two total).
+RUN: %link %linkopts %t1.o -T %p/Inputs/three_entries.t -Wlinker-script -o %t.warn7.out 2>&1 | \
+RUN:   %filecheck %s --check-prefix=WARN7
+#WARN7-COUNT-2: Warning: multiple entry points specified; the last one takes effect
+#WARN7-NOT: Warning: multiple entry points specified; the last one takes effect
+#END_TEST

test/Common/standalone/linkerscript/EntryCmd/Inputs/three_entries.t

@@ -0,0 +1,9 @@
+ENTRY(bar)
+ENTRY(foo)
+ENTRY(bar)
+SECTIONS {
+  . = 0x1000;
+  .bar : { *(.text.bar) }
+  . = 0x2000;
+  .foo : { *(.text.foo) }
+}