[clang] functions with the 'const' or 'pure' attribute must always return.

[jeroen.dobbelaere]

Summary

As described in

• https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-pure-function-attribute
• https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-const-function-attribute

An __attribute__((pure)) function must always return, as well as an __attribute__((const)) function.

[jdoerfert]

LGTM

clang/lib/CodeGen/CGCall.cpp

if (const FunctionDecl *Fn = dyn_cast<FunctionDecl>(TargetDecl)) {
       if (AttrOnCallSite && TargetDecl->hasAttr<NoMergeAttr>())
         FuncAttrs.addAttribute(llvm::Attribute::NoMerge);
     }
 
     // 'const', 'pure' and 'noalias' attributed functions are also nounwind.
     if (TargetDecl->hasAttr<ConstAttr>()) {
       FuncAttrs.addAttribute(llvm::Attribute::ReadNone);
       FuncAttrs.addAttribute(llvm::Attribute::NoUnwind);
+      // gcc specifies that 'const' functions have greater restrictions than
+      // 'pure' functions, so they also cannot have infinite loops.
+      FuncAttrs.addAttribute(llvm::Attribute::WillReturn);
     } else if (TargetDecl->hasAttr<PureAttr>()) {
       FuncAttrs.addAttribute(llvm::Attribute::ReadOnly);
       FuncAttrs.addAttribute(llvm::Attribute::NoUnwind);
+      // gcc specifies that 'pure' functions cannot have infinite loops.
+      FuncAttrs.addAttribute(llvm::Attribute::WillReturn);
     } else if (TargetDecl->hasAttr<NoAliasAttr>()) {
       FuncAttrs.addAttribute(llvm::Attribute::ArgMemOnly);
       FuncAttrs.addAttribute(llvm::Attribute::NoUnwind);
     }
     if (TargetDecl->hasAttr<RestrictAttr>())
       RetAttrs.addAttribute(llvm::Attribute::NoAlias);
     if (TargetDecl->hasAttr<ReturnsNonNullAttr>() &&
         !CodeGenOpts.NullPointerIsValid)

clang/test/CodeGen/complex-builtins.c

// HAS_ERRNO: declare { x86_fp80, x86_fp80 } @clogl({ x86_fp80, x86_fp80 }* byval({ x86_fp80, x86_fp80 }) align 16) [[NOT_READNONE]]
 
   __builtin_cproj(f);      __builtin_cprojf(f);     __builtin_cprojl(f); 
 
 // NO__ERRNO: declare { double, double } @cproj(double, double) [[READNONE]]
 // NO__ERRNO: declare <2 x float> @cprojf(<2 x float>) [[READNONE]]
 // NO__ERRNO: declare { x86_fp80, x86_fp80 } @cprojl({ x86_fp80, x86_fp80 }* byval({ x86_fp80, x86_fp80 }) align 16) [[NOT_READNONE]]
 // HAS_ERRNO: declare { double, double } @cproj(double, double) [[READNONE:#[0-9]+]]
 // HAS_ERRNO: declare <2 x float> @cprojf(<2 x float>) [[READNONE]]
-// HAS_ERRNO: declare { x86_fp80, x86_fp80 } @cprojl({ x86_fp80, x86_fp80 }* byval({ x86_fp80, x86_fp80 }) align 16) [[NOT_READNONE]]
+// HAS_ERRNO: declare { x86_fp80, x86_fp80 } @cprojl({ x86_fp80, x86_fp80 }* byval({ x86_fp80, x86_fp80 }) align 16) [[WILLRETURN_NOT_READNONE:#[0-9]+]]
 
   __builtin_cpow(f,f);       __builtin_cpowf(f,f);      __builtin_cpowl(f,f);
 
 // NO__ERRNO: declare { double, double } @cpow(double, double, double, double) [[READNONE]]
 // NO__ERRNO: declare <2 x float> @cpowf(<2 x float>, <2 x float>) [[READNONE]]
 // NO__ERRNO: declare { x86_fp80, x86_fp80 } @cpowl({ x86_fp80, x86_fp80 }* byval({ x86_fp80, x86_fp80 }) align 16, { x86_fp80, x86_fp80 }* byval({ x86_fp80, x86_fp80 }) align 16) [[NOT_READNONE]]
 // HAS_ERRNO: declare { double, double } @cpow(double, double, double, double) [[NOT_READNONE]]
 // HAS_ERRNO: declare <2 x float> @cpowf(<2 x float>, <2 x float>) [[NOT_READNONE]]

 // HAS_ERRNO: declare { x86_fp80, x86_fp80 } @ctanhl({ x86_fp80, x86_fp80 }* byval({ x86_fp80, x86_fp80 }) align 16) [[NOT_READNONE]]
 };
 
 // NO__ERRNO: attributes [[READNONE]] = { {{.*}}readnone{{.*}} }
 // NO__ERRNO: attributes [[NOT_READNONE]] = { nounwind {{.*}} }
 
 // HAS_ERRNO: attributes [[NOT_READNONE]] = { nounwind {{.*}} }
 // HAS_ERRNO: attributes [[READNONE]] = { {{.*}}readnone{{.*}} }
+// HAS_ERRNO: attributes [[WILLRETURN_NOT_READNONE]] = { nounwind willreturn {{.*}} }

clang/test/CodeGen/complex-libcalls.c

// HAS_ERRNO: declare { x86_fp80, x86_fp80 } @clogl({ x86_fp80, x86_fp80 }* byval({ x86_fp80, x86_fp80 }) align 16) [[NOT_READNONE]]
 
   cproj(f);      cprojf(f);     cprojl(f); 
 
 // NO__ERRNO: declare { double, double } @cproj(double, double) [[READNONE]]
 // NO__ERRNO: declare <2 x float> @cprojf(<2 x float>) [[READNONE]]
 // NO__ERRNO: declare { x86_fp80, x86_fp80 } @cprojl({ x86_fp80, x86_fp80 }* byval({ x86_fp80, x86_fp80 }) align 16) [[NOT_READNONE]]
 // HAS_ERRNO: declare { double, double } @cproj(double, double) [[READNONE:#[0-9]+]]
 // HAS_ERRNO: declare <2 x float> @cprojf(<2 x float>) [[READNONE]]
-// HAS_ERRNO: declare { x86_fp80, x86_fp80 } @cprojl({ x86_fp80, x86_fp80 }* byval({ x86_fp80, x86_fp80 }) align 16) [[NOT_READNONE]]
+// HAS_ERRNO: declare { x86_fp80, x86_fp80 } @cprojl({ x86_fp80, x86_fp80 }* byval({ x86_fp80, x86_fp80 }) align 16) [[WILLRETURN_NOT_READNONE:#[0-9]+]]
 
   cpow(f,f);       cpowf(f,f);      cpowl(f,f);
 
 // NO__ERRNO: declare { double, double } @cpow(double, double, double, double) [[READNONE]]
 // NO__ERRNO: declare <2 x float> @cpowf(<2 x float>, <2 x float>) [[READNONE]]
 // NO__ERRNO: declare { x86_fp80, x86_fp80 } @cpowl({ x86_fp80, x86_fp80 }* byval({ x86_fp80, x86_fp80 }) align 16, { x86_fp80, x86_fp80 }* byval({ x86_fp80, x86_fp80 }) align 16) [[NOT_READNONE]]
 // HAS_ERRNO: declare { double, double } @cpow(double, double, double, double) [[NOT_READNONE]]
 // HAS_ERRNO: declare <2 x float> @cpowf(<2 x float>, <2 x float>) [[NOT_READNONE]]

 // HAS_ERRNO: declare { x86_fp80, x86_fp80 } @ctanhl({ x86_fp80, x86_fp80 }* byval({ x86_fp80, x86_fp80 }) align 16) [[NOT_READNONE]]
 };
 
 // NO__ERRNO: attributes [[READNONE]] = { {{.*}}readnone{{.*}} }
 // NO__ERRNO: attributes [[NOT_READNONE]] = { nounwind {{.*}} }
 
 // HAS_ERRNO: attributes [[NOT_READNONE]] = { nounwind {{.*}} }
 // HAS_ERRNO: attributes [[READNONE]] = { {{.*}}readnone{{.*}} }
+// HAS_ERRNO: attributes [[WILLRETURN_NOT_READNONE]] = { nounwind willreturn {{.*}} }

clang/test/CodeGen/function-attributes.c

 }
 
 // CHECK: attributes [[NUW]] = { nounwind optsize{{.*}} }
 // CHECK: attributes [[AI]] = { alwaysinline nounwind optsize{{.*}} }
 // CHECK: attributes [[NUW_OS_RN]] = { nounwind optsize readnone{{.*}} }
 // CHECK: attributes [[SR]] = { nounwind optsize{{.*}} "stackrealign"{{.*}} }
 // CHECK: attributes [[RT]] = { nounwind optsize returns_twice{{.*}} }
 // CHECK: attributes [[NR]] = { noreturn optsize }
-// CHECK: attributes [[NUW_RN]] = { nounwind optsize readnone }
+// CHECK: attributes [[NUW_RN]] = { nounwind optsize readnone willreturn }
 // CHECK: attributes [[RT_CALL]] = { optsize returns_twice }

clang/test/CodeGenCXX/2009-05-04-PureConstNounwind.cpp

int f(void) {
   return c() + p() + t();
 }
 
 // CHECK: declare i32 @_Z1cv() [[NUW_RN:#[0-9]+]]
 // CHECK: declare i32 @_Z1pv() [[NUW_RO:#[0-9]+]]
 // CHECK: declare i32 @_Z1tv() [[TF2:#[0-9]+]]
 
 // CHECK: attributes [[TF]] = { {{.*}} }
-// CHECK: attributes [[NUW_RN]] = { nounwind readnone{{.*}} }
-// CHECK: attributes [[NUW_RO]] = { nounwind readonly{{.*}} }
+// CHECK: attributes [[NUW_RN]] = { nounwind readnone willreturn{{.*}} }
+// CHECK: attributes [[NUW_RO]] = { nounwind readonly willreturn{{.*}} }
 // CHECK: attributes [[TF2]] = { {{.*}} }
-// CHECK: attributes [[NUW_RN_CALL]] = { nounwind readnone }
-// CHECK: attributes [[NUW_RO_CALL]] = { nounwind readonly }
+// CHECK: attributes [[NUW_RN_CALL]] = { nounwind readnone willreturn }
+// CHECK: attributes [[NUW_RO_CALL]] = { nounwind readonly willreturn }

clang/test/Sema/libbuiltins-ctype-powerpc64.c

 // CHECK: declare signext i32 @ispunct(i32 signext) [[NUW_RO:#[0-9]+]]
 // CHECK: declare signext i32 @isspace(i32 signext) [[NUW_RO:#[0-9]+]]
 // CHECK: declare signext i32 @isupper(i32 signext) [[NUW_RO:#[0-9]+]]
 // CHECK: declare signext i32 @isxdigit(i32 signext) [[NUW_RO:#[0-9]+]]
 // CHECK: declare signext i32 @tolower(i32 signext) [[NUW_RO:#[0-9]+]]
 // CHECK: declare signext i32 @toupper(i32 signext) [[NUW_RO:#[0-9]+]]
 
 // CHECK: attributes [[NUW_RO]] = { nounwind readonly{{.*}} }
-// CHECK: attributes [[NUW_RO_CALL]] = { nounwind readonly }
+// CHECK: attributes [[NUW_RO_CALL]] = { nounwind readonly willreturn }

clang/test/Sema/libbuiltins-ctype-x86_64.c

 // CHECK: declare i32 @ispunct(i32) [[NUW_RO:#[0-9]+]]
 // CHECK: declare i32 @isspace(i32) [[NUW_RO:#[0-9]+]]
 // CHECK: declare i32 @isupper(i32) [[NUW_RO:#[0-9]+]]
 // CHECK: declare i32 @isxdigit(i32) [[NUW_RO:#[0-9]+]]
 // CHECK: declare i32 @tolower(i32) [[NUW_RO:#[0-9]+]]
 // CHECK: declare i32 @toupper(i32) [[NUW_RO:#[0-9]+]]
 
 // CHECK: attributes [[NUW_RO]] = { nounwind readonly{{.*}} }
-// CHECK: attributes [[NUW_RO_CALL]] = { nounwind readonly }
+// CHECK: attributes [[NUW_RO_CALL]] = { nounwind readonly willreturn }