[clang] Add size filter for stack auto init (#74777)

Add a clang flag, "-ftrivial-auto-var-init-max-size=" so that clang skips auto-init a variable if the auto-init memset size exceeds the flag setting (in bytes). Note that this skipping doesn't apply to runtime-sized variables like VLA. Considerations: "__attribute__((uninitialized))" can be used to manually opt variables out. However, there are thousands of large variables (e.g., >=1KB, most of them are arrays and used as buffers) in big codebase. Manually opting them out one by one is not efficient.
llvm · Jan 18, 2024 · a58ad3e · a58ad3e
1 parent fa6c3df
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diff --git a/clang/include/clang/Basic/DiagnosticDriverKinds.td b/clang/include/clang/Basic/DiagnosticDriverKinds.td
@@ -656,6 +656,13 @@ def err_drv_trivial_auto_var_init_stop_after_missing_dependency : Error<
 def err_drv_trivial_auto_var_init_stop_after_invalid_value : Error<
   "'-ftrivial-auto-var-init-stop-after=*' only accepts positive integers">;
 
+def err_drv_trivial_auto_var_init_max_size_missing_dependency : Error<
+  "'-ftrivial-auto-var-init-max-size=*' is used without "
+  "'-ftrivial-auto-var-init=zero' or '-ftrivial-auto-var-init=pattern'">;
+
+def err_drv_trivial_auto_var_init_max_size_invalid_value : Error<
+  "'-ftrivial-auto-var-init-max-size=*' only accepts positive integers (in bytes)">;
+
 def warn_drv_msp430_hwmult_unsupported : Warning<
   "the given MCU does not support hardware multiply, but '-mhwmult' is set to "
   "%0">, InGroup<InvalidCommandLineArgument>;

diff --git a/clang/include/clang/Basic/LangOptions.def b/clang/include/clang/Basic/LangOptions.def
@@ -378,6 +378,8 @@ ENUM_LANGOPT(TrivialAutoVarInit, TrivialAutoVarInitKind, 2, TrivialAutoVarInitKi
              "trivial automatic variable initialization")
 VALUE_LANGOPT(TrivialAutoVarInitStopAfter, 32, 0,
              "stop trivial automatic variable initialization after the specified number of instances. Must be greater than 0.")
+VALUE_LANGOPT(TrivialAutoVarInitMaxSize, 32, 0,
+             "stop trivial automatic variable initialization if var size exceeds the specified size (in bytes). Must be greater than 0.")
 ENUM_LANGOPT(SignedOverflowBehavior, SignedOverflowBehaviorTy, 2, SOB_Undefined,
              "signed integer overflow handling")
 ENUM_LANGOPT(ThreadModel  , ThreadModelKind, 2, ThreadModelKind::POSIX, "Thread Model")

diff --git a/clang/include/clang/Driver/Options.td b/clang/include/clang/Driver/Options.td
@@ -3664,6 +3664,10 @@ def ftrivial_auto_var_init_stop_after : Joined<["-"], "ftrivial-auto-var-init-st
   Visibility<[ClangOption, CC1Option, CLOption, DXCOption]>,
   HelpText<"Stop initializing trivial automatic stack variables after the specified number of instances">,
   MarshallingInfoInt<LangOpts<"TrivialAutoVarInitStopAfter">>;
+def ftrivial_auto_var_init_max_size : Joined<["-"], "ftrivial-auto-var-init-max-size=">, Group<f_Group>,
+  Visibility<[ClangOption, CC1Option, CLOption, DXCOption]>,
+  HelpText<"Stop initializing trivial automatic stack variables if var size exceeds the specified number of instances (in bytes)">,
+  MarshallingInfoInt<LangOpts<"TrivialAutoVarInitMaxSize">>;
 def fstandalone_debug : Flag<["-"], "fstandalone-debug">, Group<f_Group>,
   Visibility<[ClangOption, CLOption, DXCOption]>,
   HelpText<"Emit full debug info for all types used by the program">;

diff --git a/clang/lib/CodeGen/CGDecl.cpp b/clang/lib/CodeGen/CGDecl.cpp
@@ -1759,20 +1759,34 @@ void CodeGenFunction::emitZeroOrPatternForAutoVarInit(QualType type,
                                                       const VarDecl &D,
                                                       Address Loc) {
   auto trivialAutoVarInit = getContext().getLangOpts().getTrivialAutoVarInit();
+  auto trivialAutoVarInitMaxSize =
+      getContext().getLangOpts().TrivialAutoVarInitMaxSize;
   CharUnits Size = getContext().getTypeSizeInChars(type);
   bool isVolatile = type.isVolatileQualified();
   if (!Size.isZero()) {
+    // We skip auto-init variables by their alloc size. Take this as an example:
+    // "struct Foo {int x; char buff[1024];}" Assume the max-size flag is 1023.
+    // All Foo type variables will be skipped. Ideally, we only skip the buff
+    // array and still auto-init X in this example.
+    // TODO: Improve the size filtering to by member size.
+    auto allocSize = CGM.getDataLayout().getTypeAllocSize(Loc.getElementType());
     switch (trivialAutoVarInit) {
     case LangOptions::TrivialAutoVarInitKind::Uninitialized:
       llvm_unreachable("Uninitialized handled by caller");
     case LangOptions::TrivialAutoVarInitKind::Zero:
       if (CGM.stopAutoInit())
         return;
+      if (trivialAutoVarInitMaxSize > 0 &&
+          allocSize > trivialAutoVarInitMaxSize)
+        return;
       emitStoresForZeroInit(CGM, D, Loc, isVolatile, Builder);
       break;
     case LangOptions::TrivialAutoVarInitKind::Pattern:
       if (CGM.stopAutoInit())
         return;
+      if (trivialAutoVarInitMaxSize > 0 &&
+          allocSize > trivialAutoVarInitMaxSize)
+        return;
       emitStoresForPatternInit(CGM, D, Loc, isVolatile, Builder);
       break;
     }

diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -3535,6 +3535,20 @@ static void RenderTrivialAutoVarInitOptions(const Driver &D,
     CmdArgs.push_back(
         Args.MakeArgString("-ftrivial-auto-var-init-stop-after=" + Val));
   }
+
+  if (Arg *A = Args.getLastArg(options::OPT_ftrivial_auto_var_init_max_size)) {
+    if (!Args.hasArg(options::OPT_ftrivial_auto_var_init) ||
+        StringRef(
+            Args.getLastArg(options::OPT_ftrivial_auto_var_init)->getValue()) ==
+            "uninitialized")
+      D.Diag(diag::err_drv_trivial_auto_var_init_max_size_missing_dependency);
+    A->claim();
+    StringRef Val = A->getValue();
+    if (std::stoi(Val.str()) <= 0)
+      D.Diag(diag::err_drv_trivial_auto_var_init_max_size_invalid_value);
+    CmdArgs.push_back(
+        Args.MakeArgString("-ftrivial-auto-var-init-max-size=" + Val));
+  }
 }
 
 static void RenderOpenCLOptions(const ArgList &Args, ArgStringList &CmdArgs,

diff --git a/clang/test/CodeGenCXX/auto-var-init-max-size.cpp b/clang/test/CodeGenCXX/auto-var-init-max-size.cpp
@@ -0,0 +1,83 @@
+// Pattern related max size tests: 1, 1024, 4096
+// RUN: %clang_cc1 -triple x86_64-unknown-unknown -ftrivial-auto-var-init=pattern -ftrivial-auto-var-init-max-size=1 %s -emit-llvm -o - | FileCheck -check-prefix=PATTERN-COMMON -check-prefix=PATTERN-MAX-1 %s
+// RUN: %clang_cc1 -triple x86_64-unknown-unknown -ftrivial-auto-var-init=pattern -ftrivial-auto-var-init-max-size=1024 %s -emit-llvm -o - | FileCheck -check-prefix=PATTERN-COMMON -check-prefix=PATTERN-MAX-1024 %s
+// RUN: %clang_cc1 -triple x86_64-unknown-unknown -ftrivial-auto-var-init=pattern -ftrivial-auto-var-init-max-size=4096 %s -emit-llvm -o - | FileCheck -check-prefix=PATTERN-COMMON -check-prefix=PATTERN-MAX-4096 %s
+//
+// Zero related max size tests: 1, 1024, 4096
+// RUN: %clang_cc1 -triple x86_64-unknown-unknown -ftrivial-auto-var-init=zero -ftrivial-auto-var-init-max-size=1 %s -emit-llvm -o - | FileCheck -check-prefix=ZERO-COMMON -check-prefix=ZERO-MAX-1 %s
+// RUN: %clang_cc1 -triple x86_64-unknown-unknown -ftrivial-auto-var-init=zero -ftrivial-auto-var-init-max-size=1024 %s -emit-llvm -o - | FileCheck -check-prefix=ZERO-COMMON -check-prefix=ZERO-MAX-1024 %s
+// RUN: %clang_cc1 -triple x86_64-unknown-unknown -ftrivial-auto-var-init=zero -ftrivial-auto-var-init-max-size=4096 %s -emit-llvm -o - | FileCheck -check-prefix=ZERO-COMMON -check-prefix=ZERO-MAX-4096 %s
+
+struct Foo {
+    int x; // we should try to make sure X is initialized.
+    char buff[1024];  // this one is fine to skip
+};
+
+int foo(unsigned n) {
+  bool var_size_1;
+  long var_size_8 = 123;
+  void *var_size_8p;
+  int var_size_1024[256];
+  Foo var_size_1028;
+  int var_size_4096[1024];
+  // VLA, non-constant size
+  int var_vla[n];
+  // builtin, non-constant size
+  var_size_8p = __builtin_alloca(sizeof(unsigned long long) * n);
+  // There are 8 variables: var_size_1, var_size_8, var_size_8p, var_size_1024,
+  // var_size_1028, var_size_4096, var_vla, and a builtin anonymous var ("%5").
+  // - COMMON (auto-init regardless of the max size): "var_vla", and "%5"
+  // - Max size 1: "var_size_1"
+  // - Max size 1024: "var_size_1", "var_size_8", "var_size_8p", "var_size_1024"
+  // - Max size 4096: "var_size_1", "var_size_8", "var_size_8p", "var_size_1024", "var_size_1028", "var_size_4096"
+  //
+  // PATTERN-MAX-1: store i8 -86, ptr %var_size_1, align 1, !annotation [[AUTO_INIT:!.+]]
+  // PATTERN-MAX-1-NOT: store i64 -6148914691236517206, ptr %var_size_8, align 8, !annotation [[AUTO_INIT:!.+]]
+  // PATTERN-MAX-1-NOT: store ptr inttoptr (i64 -6148914691236517206 to ptr), ptr %var_size_8p, align 8, !annotation [[AUTO_INIT:!.+]]
+  // PATTERN-MAX-1-NOT: call void @llvm.memset.p0.i64(ptr align 16 %var_size_1024, i8 -86, i64 1024, i1 false), !annotation [[AUTO_INIT:!.+]]
+  // PATTERN-MAX-1-NOT: call void @llvm.memset.p0.i64(ptr align 4 %var_size_1028, i8 -86, i64 1028, i1 false), !annotation [[AUTO_INIT:!.+]]
+  // PATTERN-MAX-1-NOT: call void @llvm.memset.p0.i64(ptr align 16 %var_size_4096, i8 -86, i64 4096, i1 false), !annotation [[AUTO_INIT:!.+]]
+
+  // PATTERN-MAX-1024: store i8 -86, ptr %var_size_1, align 1, !annotation [[AUTO_INIT:!.+]]
+  // PATTERN-MAX-1024: store i64 -6148914691236517206, ptr %var_size_8, align 8, !annotation [[AUTO_INIT:!.+]]
+  // PATTERN-MAX-1024: store ptr inttoptr (i64 -6148914691236517206 to ptr), ptr %var_size_8p, align 8, !annotation [[AUTO_INIT:!.+]]
+  // PATTERN-MAX-1024: call void @llvm.memset.p0.i64(ptr align 16 %var_size_1024, i8 -86, i64 1024, i1 false), !annotation [[AUTO_INIT:!.+]]
+  // PATTERN-MAX-1024-NOT: call void @llvm.memset.p0.i64(ptr align 4 %var_size_1028, i8 -86, i64 1028, i1 false), !annotation [[AUTO_INIT:!.+]]
+  // PATTERN-MAX-1024-NOT: call void @llvm.memset.p0.i64(ptr align 16 %var_size_4096, i8 -86, i64 4096, i1 false), !annotation [[AUTO_INIT:!.+]]
+
+  // PATTERN-MAX-4096: store i8 -86, ptr %var_size_1, align 1, !annotation [[AUTO_INIT:!.+]]
+  // PATTERN-MAX-4096: store i64 -6148914691236517206, ptr %var_size_8, align 8, !annotation [[AUTO_INIT:!.+]]
+  // PATTERN-MAX-4096: store ptr inttoptr (i64 -6148914691236517206 to ptr), ptr %var_size_8p, align 8, !annotation [[AUTO_INIT:!.+]]
+  // PATTERN-MAX-4096: call void @llvm.memset.p0.i64(ptr align 16 %var_size_1024, i8 -86, i64 1024, i1 false), !annotation [[AUTO_INIT:!.+]]
+  // PATTERN-MAX-4096: call void @llvm.memset.p0.i64(ptr align 4 %var_size_1028, i8 -86, i64 1028, i1 false), !annotation [[AUTO_INIT:!.+]]
+  // PATTERN-MAX-4096: call void @llvm.memset.p0.i64(ptr align 16 %var_size_4096, i8 -86, i64 4096, i1 false), !annotation [[AUTO_INIT:!.+]]
+
+  // PATTERN-COMMON: call void @llvm.memcpy.p0.p0.i64(ptr align 4 %vla.cur, ptr align 4 @__const._Z3fooj.var_vla, i64 4, i1 false), !annotation [[AUTO_INIT:!.+]]
+  // PATTERN-COMMON: call void @llvm.memset.p0.i64(ptr align 16 %5, i8 -86, i64 %mul, i1 false), !annotation [[AUTO_INIT:!.+]]
+
+  // ZERO-MAX-1: store i8 0, ptr %var_size_1, align 1, !annotation [[AUTO_INIT:!.+]]
+  // ZERO-MAX-1-NOT: store i64 0, ptr %var_size_8, align 8, !annotation [[AUTO_INIT:!.+]]
+  // ZERO-MAX-1-NOT: store ptr null, ptr %var_size_8p, align 8, !annotation [[AUTO_INIT:!.+]]
+  // ZERO-MAX-1-NOT: call void @llvm.memset.p0.i64(ptr align 16 %var_size_1024, i8 0, i64 1024, i1 false), !annotation [[AUTO_INIT:!.+]]
+  // ZERO-MAX-1-NOT: call void @llvm.memset.p0.i64(ptr align 4 %var_size_1028, i8 0, i64 1028, i1 false), !annotation [[AUTO_INIT:!.+]]
+  // ZERO-MAX-1-NOT: call void @llvm.memset.p0.i64(ptr align 16 %var_size_4096, i8 0, i64 4096, i1 false), !annotation [[AUTO_INIT:!.+]]
+
+  // ZERO-MAX-1024: store i8 0, ptr %var_size_1, align 1, !annotation [[AUTO_INIT:!.+]]
+  // ZERO-MAX-1024: store i64 0, ptr %var_size_8, align 8, !annotation [[AUTO_INIT:!.+]]
+  // ZERO-MAX-1024: store ptr null, ptr %var_size_8p, align 8, !annotation [[AUTO_INIT:!.+]]
+  // ZERO-MAX-1024: call void @llvm.memset.p0.i64(ptr align 16 %var_size_1024, i8 0, i64 1024, i1 false), !annotation [[AUTO_INIT:!.+]]
+  // ZERO-MAX-1024-NOT: call void @llvm.memset.p0.i64(ptr align 4 %var_size_1028, i8 0, i64 1028, i1 false), !annotation [[AUTO_INIT:!.+]]
+  // ZERO-MAX-1024-NOT: call void @llvm.memset.p0.i64(ptr align 16 %var_size_4096, i8 0, i64 4096, i1 false), !annotation [[AUTO_INIT:!.+]]
+
+  // ZERO-MAX-4096: store i8 0, ptr %var_size_1, align 1, !annotation [[AUTO_INIT:!.+]]
+  // ZERO-MAX-4096: store i64 0, ptr %var_size_8, align 8, !annotation [[AUTO_INIT:!.+]]
+  // ZERO-MAX-4096: store ptr null, ptr %var_size_8p, align 8, !annotation [[AUTO_INIT:!.+]]
+  // ZERO-MAX-4096: call void @llvm.memset.p0.i64(ptr align 16 %var_size_1024, i8 0, i64 1024, i1 false), !annotation [[AUTO_INIT:!.+]]
+  // ZERO-MAX-4096: call void @llvm.memset.p0.i64(ptr align 4 %var_size_1028, i8 0, i64 1028, i1 false), !annotation [[AUTO_INIT:!.+]]
+  // ZERO-MAX-4096: call void @llvm.memset.p0.i64(ptr align 16 %var_size_4096, i8 0, i64 4096, i1 false), !annotation [[AUTO_INIT:!.+]]
+
+  // ZERO-COMMON: call void @llvm.memset.p0.i64(ptr align 16 %vla, i8 0, i64 %3, i1 false), !annotation [[AUTO_INIT:!.+]]
+  // ZERO-COMMON: call void @llvm.memset.p0.i64(ptr align 16 %5, i8 0, i64 %mul, i1 false), !annotation [[AUTO_INIT:!.+]]
+
+  return 0;
+}
diff --git a/clang/test/Driver/clang_f_opts.c b/clang/test/Driver/clang_f_opts.c
@@ -585,6 +585,19 @@
 // CHECK-TRIVIAL-PATTERN-STOP-AFTER-INVALID-VALUE: only accepts positive integers
 // CHECK-TRIVIAL-ZERO-STOP-AFTER-INVALID-VALUE: only accepts positive integers
 
+// RUN: %clang -### -S -ftrivial-auto-var-init=pattern -ftrivial-auto-var-init-max-size=1024 %s 2>&1 | FileCheck -check-prefix=CHECK-TRIVIAL-PATTERN-MAX-SIZE %s
+// RUN: %clang -### -S -ftrivial-auto-var-init=zero -ftrivial-auto-var-init-max-size=1024 %s 2>&1 | FileCheck -check-prefix=CHECK-TRIVIAL-ZERO-MAX-SIZE %s
+// RUN: not %clang -### -S -ftrivial-auto-var-init-max-size=1024 %s 2>&1 | FileCheck -check-prefix=CHECK-TRIVIAL-MAX-SIZE-MISSING-DEPENDENCY %s
+// RUN: not %clang -### -S -ftrivial-auto-var-init=pattern -ftrivial-auto-var-init-max-size=0 %s 2>&1 | FileCheck -check-prefix=CHECK-TRIVIAL-PATTERN-MAX-SIZE-INVALID-VALUE %s
+// RUN: not %clang -### -S -ftrivial-auto-var-init=zero -ftrivial-auto-var-init-max-size=0 %s 2>&1 | FileCheck -check-prefix=CHECK-TRIVIAL-ZERO-MAX-SIZE-INVALID-VALUE %s
+// CHECK-TRIVIAL-PATTERN-MAX-SIZE-NOT: is used without '-ftrivial-auto-var-init'
+// CHECK-TRIVIAL-PATTERN-MAX-SIZE-NOT: only accepts positive integers (in bytes)
+// CHECK-TRIVIAL-ZERO-MAX-SIZE-NOT: is used without '-ftrivial-auto-var-init'
+// CHECK-TRIVIAL-ZERO-MAX-SIZE-NOT: only accepts positive integers (in bytes)
+// CHECK-TRIVIAL-MAX-SIZE-MISSING-DEPENDENCY: used without '-ftrivial-auto-var-init=zero' or
+// CHECK-TRIVIAL-PATTERN-MAX-SIZE-INVALID-VALUE: only accepts positive integers (in bytes)
+// CHECK-TRIVIAL-ZERO-MAX-SIZE-INVALID-VALUE: only accepts positive integers (in bytes)
+
 // RUN: %clang -### -S -fno-temp-file %s 2>&1 | FileCheck -check-prefix=CHECK-NO-TEMP-FILE %s
 // CHECK-NO-TEMP-FILE: "-fno-temp-file"