[clang] Add clang warning if fp exception functions are called without appropriate flags/pragmas (#128239) (PR #187860)

[via cfe-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Marcos Ramirez Joos (maarcosrmz)

<details>
<summary>Changes</summary>

This pull request relates to issue (#<!-- -->128239). 

In the first commit, warnings were only added for 5 out of the 11 functions (see [this](https://linux.die.net/man/3/fetestexcept)):
* `int feclearexcept(int excepts);`
* `int feraiseexcept(int excepts);`
* `int fetestexcept(int excepts);`
* `int fegetround(void);`
* `int fesetround(int rounding_mode);`

The changes were tested locally both with and without `#pragma STDC FENV_ACCESS ON` as well as compiling with and without the compiler flag '-ffp-exception-behavior=maytrap', resulting in the expected behaviour described by the issue. Sample output (pragma/flag not set):
```sh
$ ./build/bin/clang test.c -lm
test.c:8:3: warning: 'feclearexcept' used without enabling floating-point exception behavior; use 'pragma STDCFENV_ACCESS ON' or compile with '-ffp-exception-behavior=maytrap'
      [-Wfenv-access]
    8 |   feclearexcept(FE_INVALID);
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~
test.c:10:3: warning: 'feraiseexcept' used without enabling floating-point exception behavior; use 'pragma STDCFENV_ACCESS ON' or compile with '-ffp-exception-behavior=maytrap'
      [-Wfenv-access]
   10 |   feraiseexcept(FE_INVALID);
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~
test.c:12:3: warning: 'fetestexcept' used without enabling floating-point exception behavior; use 'pragma STDCFENV_ACCESS ON' or compile with '-ffp-exception-behavior=maytrap'
      [-Wfenv-access]
   12 |   fetestexcept(FE_INVALID);
      |   ^~~~~~~~~~~~~~~~~~~~~~~~
test.c:13:11: warning: 'fegetround' used without enabling floating-point exception behavior; use 'pragma STDCFENV_ACCESS ON' or compile with '-ffp-exception-behavior=maytrap'
      [-Wfenv-access]
   13 |   int i = fegetround();
      |           ^~~~~~~~~~~~
test.c:15:3: warning: 'fesetround' used without enabling floating-point exception behavior; use 'pragma STDCFENV_ACCESS ON' or compile with '-ffp-exception-behavior=maytrap'
      [-Wfenv-access]
   15 |   fesetround(0);
      |   ^~~~~~~~~~~~~
5 warnings generated.
```


**Note**: Support for the remaining 6 functions was not added due to their prototypes including non-basic types, e.g. `fenv_t`. Adding these functions to `clang/include/clang/Basic/Builtins.td` results in errors like:

```sh
$ cmake --build build                       
[1/1045] Building Builtins.inc...
FAILED: [code=1] tools/clang/include/clang/Basic/Builtins.inc /home/<user>/dev/oss/llvm/llvm-project/build/tools/clang/include/clang/Basic/Builtins.inc 
cd /home/<user>/dev/oss/llvm/llvm-project/build/tools/clang/include/clang/Basic && /home/<user>/dev/oss/llvm/llvm-project/build/bin/clang-tblgen -gen-clang-builtins -I/home/<user>/dev/oss/llvm/llvm-project/clang/include/clang/Basic -I/home/<user>/dev/oss/llvm/llvm-project/clang/include -I/home/<user>/dev/oss/llvm/llvm-project/build/tools/clang/include -I/home/<user>/dev/oss/llvm/llvm-project/build/include -I/home/<user>/dev/oss/llvm/llvm-project/llvm/include /home/<user>/dev/oss/llvm/llvm-project/clang/include/clang/Basic/Builtins.td --write-if-changed -o Builtins.inc -d Builtins.inc.d && /usr/bin/cmake -E cmake_transform_depfile Ninja gccdepfile /home/<user>/dev/oss/llvm/llvm-project/llvm /home/<user>/dev/oss/llvm/llvm-project/clang/include/clang/Basic /home/<user>/dev/oss/llvm/llvm-project/build /home/<user>/dev/oss/llvm/llvm-project/build/tools/clang/include/clang/Basic /home/<user>/dev/oss/llvm/llvm-project/build/tools/clang/include/clang/Basic/Builtins.inc.d /home/<user>/dev/oss/llvm/llvm-project/build/CMakeFiles/d/782b9558ba052deea0a4e2652d46d9bd336ed7b27f8cd46fb2ef5ed62ca0bdca.d
/home/<user>/dev/oss/llvm/llvm-project/clang/include/clang/Basic/Builtins.td:4439:7: error: Unknown Type: fenv_t
  let Prototype = "int(fenv_t*)";
      ^
[2/1045] Generating VCSRevision.h
ninja: build stopped: subcommand failed.
```

With some additional guidance helping me to make this work, I would be happy to add the warning for the remaining functions.

---
Full diff: https://github.com/llvm/llvm-project/pull/187860.diff


4 Files Affected:

- (modified) clang/include/clang/Basic/BuiltinHeaders.def (+1) 
- (modified) clang/include/clang/Basic/Builtins.td (+25) 
- (modified) clang/include/clang/Basic/DiagnosticSemaKinds.td (+5) 
- (modified) clang/lib/Sema/SemaChecking.cpp (+11) 


``````````diff
diff --git a/clang/include/clang/Basic/BuiltinHeaders.def b/clang/include/clang/Basic/BuiltinHeaders.def
index d6012a896eca9..a999d3f715636 100644
--- a/clang/include/clang/Basic/BuiltinHeaders.def
+++ b/clang/include/clang/Basic/BuiltinHeaders.def
@@ -17,6 +17,7 @@ HEADER(BLOCKS_H, "Blocks.h")
 HEADER(COMPLEX_H, "complex.h")
 HEADER(CTYPE_H, "ctype.h")
 HEADER(EMMINTRIN_H, "emmintrin.h")
+HEADER(FENV_H, "fenv.h")
 HEADER(FOUNDATION_NSOBJCRUNTIME_H, "Foundation/NSObjCRuntime.h")
 HEADER(IMMINTRIN_H, "immintrin.h")
 HEADER(INTRIN_H, "intrin.h")
diff --git a/clang/include/clang/Basic/Builtins.td b/clang/include/clang/Basic/Builtins.td
index 8e002c5b900aa..862dfd5d37049 100644
--- a/clang/include/clang/Basic/Builtins.td
+++ b/clang/include/clang/Basic/Builtins.td
@@ -4409,6 +4409,31 @@ def BlockObjectDispose : LibBuiltin<"blocks.h"> {
 }
 // FIXME: Also declare NSConcreteGlobalBlock and NSConcreteStackBlock.
 
+def FeClearExcept : LibBuiltin<"fenv.h"> {
+  let Spellings = ["feclearexcept"];
+  let Prototype = "int(int)";
+}
+
+def FeRaiseExcept : LibBuiltin<"fenv.h"> {
+  let Spellings = ["feraiseexcept"];
+  let Prototype = "int(int)";
+}
+
+def FeTestExcept : LibBuiltin<"fenv.h"> {
+  let Spellings = ["fetestexcept"];
+  let Prototype = "int(int)";
+}
+
+def FeGetRound : LibBuiltin<"fenv.h"> {
+  let Spellings = ["fegetround"];
+  let Prototype = "int()";
+}
+
+def FeSetRound : LibBuiltin<"fenv.h"> {
+  let Spellings = ["fesetround"];
+  let Prototype = "int(int)";
+}
+
 def __Addressof : LangBuiltin<"CXX_LANG"> {
   let Spellings = ["__addressof"];
   let Attributes = [FunctionWithoutBuiltinPrefix, NoThrow, Const,
diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index d4d09a8ecef36..6a4f139662e31 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -1032,6 +1032,11 @@ def err_ptrauth_indirect_goto_addrlabel_arithmetic : Error<
   "%select{subtraction|addition}0 of address-of-label expressions is not "
   "supported with ptrauth indirect gotos">;
 
+def warn_fe_access_without_fenv_access : Warning<
+  "'%0' used without enabling floating-point exception behavior; use 'pragma STDC"
+  "FENV_ACCESS ON' or compile with '-ffp-exception-behavior=maytrap'">,
+  InGroup<DiagGroup<"fenv-access">>;
+
 // __ptrauth qualifier
 def err_ptrauth_qualifier_invalid : Error<
   "%select{return type|parameter type|property}1 may not be qualified with "
diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
index 29add9d092e6b..516731ff0ca3d 100644
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -3845,6 +3845,17 @@ Sema::CheckBuiltinFunctionCall(FunctionDecl *FDecl, unsigned BuiltinID,
     if (BuiltinCountedByRef(TheCall))
       return ExprError();
     break;
+
+  case Builtin::BIfeclearexcept:
+  case Builtin::BIferaiseexcept:
+  case Builtin::BIfetestexcept:
+  case Builtin::BIfegetround:
+  case Builtin::BIfesetround:
+    if (!TheCall->getFPFeaturesInEffect(getLangOpts()).getAllowFEnvAccess() &&
+            getLangOpts().getFPExceptionMode() != LangOptions::FPE_MayTrap) {
+      Diag(TheCall->getBeginLoc(), diag::warn_fe_access_without_fenv_access) 
+          << FDecl->getName() << TheCall->getSourceRange();
+    }
   }
 
   if (getLangOpts().HLSL && HLSL().CheckBuiltinFunctionCall(BuiltinID, TheCall))

``````````

</details>


https://github.com/llvm/llvm-project/pull/187860