[clang] Reset FP options before template instantiation

[sepavloff]

Summary

AST nodes that may depend on FP options keep them as a difference
relative to the options outside the AST node. At the moment of
instantiation the FP options may be different from the default values,
defined by command-line option. In such case FP attributes would have
unexpected values. For example, the code:

template <class C> void func_01(int last, C) {
  func_01(last, int());
}
void func_02() { func_01(0, 1); }
#pragma STDC FENV_ACCESS ON

caused compiler crash, because template instantiation takes place at the
end of translation unit, where pragma STDC FENV_ACCESS is in effect. As
a result, code in the template instantiation would use constrained
intrinsics while the function does not have StrictFP attribute.

To solve this problem, FP attributes in Sema must be set to default
values, defined by command line options.

This change resolves https://github.com/llvm/llvm-project/issues/63542.

[zahiraam]

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

5093

This seems to be fixing the crash.

clang/test/CodeGen/fp-template.cpp

28

Shouldn't we be expecting a constraint add here?

[sepavloff]

Fix test

[sepavloff]

clang/test/CodeGen/fp-template.cpp

28

Yes, you are right. I incorrectly merged the fix to main branch.

Thank you!

[zahiraam]

LGTM. Thanks for the patch.

[hubert.reinterpretcast]

This patch seems to be the direct cause of a regression: https://github.com/llvm/llvm-project/issues/64605.

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

if (PatternDecl->isDefaulted()) {
     SubstQualifier(*this, PatternDecl, Function, TemplateArgs);
 
     ActOnStartOfFunctionDef(nullptr, Function);
 
     // Enter the scope of this instantiation. We don't use
     // PushDeclContext because we don't have a scope.
     Sema::ContextRAII savedContext(*this, Function);
 
+    FPFeaturesStateRAII SavedFPFeatures(*this);
+    CurFPFeatures = FPOptions(getLangOpts());
+    FpPragmaStack.CurrentValue = FPOptionsOverride();
+
     if (addInstantiatedParametersToScope(Function, PatternDecl, Scope,
                                          TemplateArgs))
       return;
 
     StmtResult Body;
     if (PatternDecl->hasSkippedBody()) {
       ActOnSkippedFunctionBody(Function);
       Body = nullptr;

clang/test/CodeGen/fp-template.cpp

 Ty templ_02(Ty x, Ty y) {
   return x + y;
 }
 
 #pragma STDC FENV_ROUND FE_UPWARD
 
 template <typename Ty>
 Ty templ_03(Ty x, Ty y) {
   return x - y;
 }
 
 #pragma STDC FENV_ROUND FE_TONEAREST
 
 float func_02(float x, float y) {
   return templ_02(x, y);
 }
 
 // CHECK-LABEL: define {{.*}} float @_Z8templ_02IfET_S0_S0_
 // CHECK:       %add = fadd float %0, %1
 
 float func_03(float x, float y) {
   return templ_03(x, y);
 }
 
 // CHECK-LABEL: define {{.*}} float @_Z8templ_03IfET_S0_S0_
 // CHECK:       call float @llvm.experimental.constrained.fsub.f32({{.*}}, metadata !"round.upward", metadata !"fpexcept.ignore")
 
 
-// This pragma sets non-default rounding mode before delayed parsing occurs. It
-// is used to check that the parsing uses FP options defined by command line
-// options or by pragma before the template definition but not by this pragma.
-#pragma STDC FENV_ROUND FE_TOWARDZERO
+#pragma STDC FENV_ROUND FE_TONEAREST
+
+namespace PR63542 {
+  template <class Compare> float stable_sort(float x, Compare) {
+    float result = x + x;
+    stable_sort(x, int());
+    return result;
+  }
+  float linkage_wrap() { return stable_sort(0.0, 1); }
+}
+
+// CHECK-LABEL: define {{.*}} float @_ZN7PR6354211stable_sortIiEEffT_(
+// CHECK:         fadd float
 
+// These pragmas set non-default FP environment before delayed parsing occurs.
+// It is used to check that the parsing uses FP options defined by command line
+// options or by pragma before the template definition but not by these pragmas.
+#pragma STDC FENV_ROUND FE_TOWARDZERO
+#pragma STDC FENV_ACCESS ON
 
 // CHECK: attributes #[[ATTR01]] = { {{.*}}strictfp