Don't emit prologues or epilogues for naked functions (PR18791, PR20028)

For naked functions with parameters, Clang would still emit stores in the prologue
that would clobber the stack, because LLVM doesn't set up a stack frame. (This
shows up in -O0 compiles, because the stores are optimized away otherwise.)

For example:

  __attribute__((naked)) int f(int x) {
    asm("movl $42, %eax");
    asm("retl");
  }

Would result in:

  _Z1fi:
  movl    12(%esp), %eax
  movl    %eax, (%esp)    <--- Oops.
  movl    $42, %eax
  retl

Differential Revision: http://reviews.llvm.org/D5183

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@217198 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/CodeGen/CGCall.cpp b/lib/CodeGen/CGCall.cpp
index ce16d3bd64c4659250ea6eba3484a8b7fd14f2a8..f0eeba06f9cbb5845d9b650416e3f162a3911dbf 100644 (file)
--- a/lib/CodeGen/CGCall.cpp
+++ b/lib/CodeGen/CGCall.cpp
@@ -1462,6 +1462,10 @@ static bool shouldAddNonNullAttr(const Decl *FD, const ParmVarDecl *PVD) {
 void CodeGenFunction::EmitFunctionProlog(const CGFunctionInfo &FI,
                                          llvm::Function *Fn,
                                          const FunctionArgList &Args) {
+  if (CurCodeDecl && CurCodeDecl->hasAttr<NakedAttr>())
+    // Naked functions don't have prologues.
+    return;
+
   // If this is an implicit-return-zero function, go ahead and
   // initialize the return value.  TODO: it might be nice to have
   // a more general mechanism for this that didn't require synthesized
@@ -1985,6 +1989,12 @@ static llvm::StoreInst *findDominatingStoreToReturnValue(CodeGenFunction &CGF) {
 void CodeGenFunction::EmitFunctionEpilog(const CGFunctionInfo &FI,
                                          bool EmitRetDbgLoc,
                                          SourceLocation EndLoc) {
+  if (CurCodeDecl && CurCodeDecl->hasAttr<NakedAttr>()) {
+    // Naked functions don't have epilogues.
+    Builder.CreateUnreachable();
+    return;
+  }
+
   // Functions with no result always return void.
   if (!ReturnValue) {
     Builder.CreateRetVoid();

diff --git a/test/CodeGen/attr-naked.c b/test/CodeGen/attr-naked.c
index c07dd8d3732a4126485fc0facab7513bd4095ced..270fc7959f4437ef76de339d489bbcf5cfbc7ee1 100644 (file)
--- a/test/CodeGen/attr-naked.c
+++ b/test/CodeGen/attr-naked.c
@@ -12,7 +12,15 @@ void t1()
 // Make sure this doesn't explode in the verifier.
 // (It doesn't really make sense, but it isn't invalid.)
 // CHECK: define void @t2() [[NAKED]] {
-__attribute((naked, always_inline)) void t2()  {
+__attribute((naked, always_inline)) void t2() {
+}
+
+// Make sure not to generate prolog or epilog for naked functions.
+__attribute((naked)) void t3(int x) {
+// CHECK: define void @t3(i32)
+// CHECK-NOT: alloca
+// CHECK-NOT: store
+// CHECK: unreachable
 }
 
 // CHECK: attributes [[NAKED]] = { naked noinline nounwind{{.*}} }