[clang] [clang][ARM] Fix warning for VFP function calls from interrupts. (PR #91870)

[Chris Copeland via cfe-commits]

https://github.com/chrisnc updated https://github.com/llvm/llvm-project/pull/91870

>From b892c59a27089b4753b7677a1ada1ee8da59301b Mon Sep 17 00:00:00 2001
From: Chris Copeland <chris at chrisnc.net>
Date: Sat, 11 May 2024 00:15:50 -0700
Subject: [PATCH 1/2] [clang][ARM] Fix warning for using VFP from interrupts.

This warning has three issues:
 - The interrupt attribute causes the function to return using an exception
   return instruction. This warning allows calls from one function with
   the interrupt attribute to another, and the diagnostic text suggests
   that not having the attribute on the callee is a problem. Actually
   making such a call will lead to a double exception return, which is
   unpredictable according to the ARM architecture manual section
   B9.1.1, "Restrictions on exception return instructions". Even on
   machines where an exception return from user/system mode is
   tolerated, if the callee's interrupt type is anything other than a
   supervisor call or secure monitor call, it will also return to a
   different address than a normal function would. For example,
   returning from an "IRQ" handler will return to lr - 4, which will
   generally result in calling the same function again.
 - The interrupt attribute currently does not cause caller-saved VFP
   registers to be saved and restored if they are used, so putting
   __attribute__((interrupt)) on a called function doesn't prevent it
   from clobbering VFP state.
 - It is part of the -Wextra diagnostic group and can't be individually
   disabled when using -Wextra, which also means the diagnostic text of
   this specific warning appears in the documentation of -Wextra.

This change addresses all three issues by instead generating a warning
for any interrupt handler where the vfp feature is enabled. The warning is
also given its own diagnostic group.

Closes #34876.
---
 clang/docs/ReleaseNotes.rst                   | 11 +++++
 .../clang/Basic/DiagnosticSemaKinds.td        |  7 +--
 clang/lib/Sema/SemaARM.cpp                    |  5 +++
 clang/lib/Sema/SemaExpr.cpp                   | 14 +-----
 clang/test/Sema/arm-interrupt-attr.c          | 45 +++----------------
 5 files changed, 27 insertions(+), 55 deletions(-)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 148ff05008552..9d007c3e28cd9 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -409,6 +409,11 @@ Modified Compiler Flags
   evaluating to ``true`` and an empty body such as ``while(1);``)
   are considered infinite, even when the ``-ffinite-loop`` flag is set.
 
+- Removed "arm interrupt calling convention" warning that was included in
+  ``-Wextra`` without its own flag.
+
+- Added ``-Warm-interrupt-vfp-clobber``, with its own warning group.
+
 Removed Compiler Flags
 -------------------------
 
@@ -569,6 +574,12 @@ Improvements to Clang's diagnostics
 - Clang no longer emits a "declared here" note for a builtin function that has no declaration in source.
   Fixes #GH93369.
 
+- For the ARM target, Clang no longer suggests adding ``__attribute__((interrupt))`` to
+  functions that are called from interrupt handlers to prevent clobbering VFP registers
+  as part of ``-Wextra`` (#GH34876). Following this suggestion leads to unpredictable
+  behavior. Instead, a new warning, ``-Warm-interrupt-vfp-clobber`` will trigger for
+  interrupt handlers with VFP enabled.
+
 Improvements to Clang's time-trace
 ----------------------------------
 
diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index 193eae3bc41d6..56598cafdd0e6 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -336,9 +336,10 @@ def warn_anyx86_excessive_regsave : Warning<
   " with attribute 'no_caller_saved_registers'"
   " or be compiled with '-mgeneral-regs-only'">,
   InGroup<DiagGroup<"excessive-regsave">>;
-def warn_arm_interrupt_calling_convention : Warning<
-   "call to function without interrupt attribute could clobber interruptee's VFP registers">,
-   InGroup<Extra>;
+def warn_arm_interrupt_vfp_clobber : Warning<
+  "interrupt service routine with vfp enabled may clobber the "
+  "interruptee's vfp state">,
+  InGroup<DiagGroup<"arm-interrupt-vfp-clobber">>;
 def warn_interrupt_attribute_invalid : Warning<
    "%select{MIPS|MSP430|RISC-V}0 'interrupt' attribute only applies to "
    "functions that have %select{no parameters|a 'void' return type}1">,
diff --git a/clang/lib/Sema/SemaARM.cpp b/clang/lib/Sema/SemaARM.cpp
index 02e68dbdb2e9d..5face34d145ae 100644
--- a/clang/lib/Sema/SemaARM.cpp
+++ b/clang/lib/Sema/SemaARM.cpp
@@ -1277,6 +1277,11 @@ void SemaARM::handleInterruptAttr(Decl *D, const ParsedAttr &AL) {
     return;
   }
 
+  const TargetInfo &TI = getASTContext().getTargetInfo();
+  if (TI.hasFeature("vfp")) {
+    Diag(D->getLocation(), diag::warn_arm_interrupt_vfp_clobber);
+  }
+
   D->addAttr(::new (getASTContext())
                  ARMInterruptAttr(getASTContext(), AL, Kind));
 }
diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
index 76145f291887c..91ddad3e83ca5 100644
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -6760,22 +6760,10 @@ ExprResult Sema::BuildResolvedCallExpr(Expr *Fn, NamedDecl *NDecl,
     return ExprError();
   }
 
-  // Interrupt handlers don't save off the VFP regs automatically on ARM,
-  // so there's some risk when calling out to non-interrupt handler functions
-  // that the callee might not preserve them. This is easy to diagnose here,
-  // but can be very challenging to debug.
-  // Likewise, X86 interrupt handlers may only call routines with attribute
+  // X86 interrupt handlers may only call routines with attribute
   // no_caller_saved_registers since there is no efficient way to
   // save and restore the non-GPR state.
   if (auto *Caller = getCurFunctionDecl()) {
-    if (Caller->hasAttr<ARMInterruptAttr>()) {
-      bool VFP = Context.getTargetInfo().hasFeature("vfp");
-      if (VFP && (!FDecl || !FDecl->hasAttr<ARMInterruptAttr>())) {
-        Diag(Fn->getExprLoc(), diag::warn_arm_interrupt_calling_convention);
-        if (FDecl)
-          Diag(FDecl->getLocation(), diag::note_callee_decl) << FDecl;
-      }
-    }
     if (Caller->hasAttr<AnyX86InterruptAttr>() ||
         Caller->hasAttr<AnyX86NoCallerSavedRegistersAttr>()) {
       const TargetInfo &TI = Context.getTargetInfo();
diff --git a/clang/test/Sema/arm-interrupt-attr.c b/clang/test/Sema/arm-interrupt-attr.c
index 3537fba8521ad..aed2c262b440e 100644
--- a/clang/test/Sema/arm-interrupt-attr.c
+++ b/clang/test/Sema/arm-interrupt-attr.c
@@ -1,52 +1,19 @@
-// RUN: %clang_cc1 %s -triple arm-apple-darwin  -target-feature +vfp2 -verify -fsyntax-only
-// RUN: %clang_cc1 %s -triple thumb-apple-darwin  -target-feature +vfp3 -verify -fsyntax-only
-// RUN: %clang_cc1 %s -triple armeb-none-eabi  -target-feature +vfp4 -verify -fsyntax-only
-// RUN: %clang_cc1 %s -triple thumbeb-none-eabi  -target-feature +neon -verify -fsyntax-only
-// RUN: %clang_cc1 %s -triple thumbeb-none-eabi -target-feature +neon -target-feature +soft-float -DSOFT -verify -fsyntax-only
+// RUN: %clang_cc1 %s -triple arm-none-eabi -DSOFT -verify -fsyntax-only
+// RUN: %clang_cc1 %s -triple arm-none-eabi -target-feature +vfp2 -verify -fsyntax-only
 
-__attribute__((interrupt(IRQ))) void foo(void) {} // expected-error {{'interrupt' attribute requires a string}}
-__attribute__((interrupt("irq"))) void foo1(void) {} // expected-warning {{'interrupt' attribute argument not supported: irq}}
 
+#ifdef SOFT
+__attribute__((interrupt("irq"))) void foo1(void) {} // expected-warning {{'interrupt' attribute argument not supported: irq}}
+__attribute__((interrupt(IRQ))) void foo(void) {} // expected-error {{'interrupt' attribute requires a string}}
 __attribute__((interrupt("IRQ", 1))) void foo2(void) {} // expected-error {{'interrupt' attribute takes no more than 1 argument}}
-
 __attribute__((interrupt("IRQ"))) void foo3(void) {}
 __attribute__((interrupt("FIQ"))) void foo4(void) {}
 __attribute__((interrupt("SWI"))) void foo5(void) {}
 __attribute__((interrupt("ABORT"))) void foo6(void) {}
 __attribute__((interrupt("UNDEF"))) void foo7(void) {}
-
 __attribute__((interrupt)) void foo8(void) {}
 __attribute__((interrupt())) void foo9(void) {}
 __attribute__((interrupt(""))) void foo10(void) {}
-
-#ifndef SOFT
-// expected-note at +2 {{'callee1' declared here}}
-#endif
-void callee1(void);
-__attribute__((interrupt("IRQ"))) void callee2(void);
-void caller1(void) {
-  callee1();
-  callee2();
-}
-
-#ifndef SOFT
-__attribute__((interrupt("IRQ"))) void caller2(void) {
-  callee1(); // expected-warning {{call to function without interrupt attribute could clobber interruptee's VFP registers}}
-  callee2();
-}
-
-void (*callee3)(void);
-__attribute__((interrupt("IRQ"))) void caller3(void) {
-  callee3(); // expected-warning {{call to function without interrupt attribute could clobber interruptee's VFP registers}}
-}
 #else
-__attribute__((interrupt("IRQ"))) void caller2(void) {
-  callee1();
-  callee2();
-}
-
-void (*callee3)(void);
-__attribute__((interrupt("IRQ"))) void caller3(void) {
-  callee3();
-}
+__attribute__((interrupt("IRQ"))) void float_irq(void); // expected-warning {{interrupt service routine with vfp enabled may clobber the interruptee's vfp state}}
 #endif

>From 9b357d2dff1ac55671e32a15d38b8876416e6187 Mon Sep 17 00:00:00 2001
From: Chris Copeland <chris at chrisnc.net>
Date: Thu, 13 Jun 2024 23:34:43 -0700
Subject: [PATCH 2/2] [clang][ARM] Emit an error when an interrupt handler is
 called.

Closes #95359.
---
 clang/docs/ReleaseNotes.rst                      |  2 +-
 clang/include/clang/Basic/DiagnosticSemaKinds.td |  2 ++
 clang/lib/Sema/SemaExpr.cpp                      | 12 +++++++++---
 3 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 9d007c3e28cd9..d0737b412a491 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -578,7 +578,7 @@ Improvements to Clang's diagnostics
   functions that are called from interrupt handlers to prevent clobbering VFP registers
   as part of ``-Wextra`` (#GH34876). Following this suggestion leads to unpredictable
   behavior. Instead, a new warning, ``-Warm-interrupt-vfp-clobber`` will trigger for
-  interrupt handlers with VFP enabled.
+  interrupt handlers with VFP enabled. Calling an interrupt handler is now an error.
 
 Improvements to Clang's time-trace
 ----------------------------------
diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index 56598cafdd0e6..d34f472383954 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -340,6 +340,8 @@ def warn_arm_interrupt_vfp_clobber : Warning<
   "interrupt service routine with vfp enabled may clobber the "
   "interruptee's vfp state">,
   InGroup<DiagGroup<"arm-interrupt-vfp-clobber">>;
+def err_arm_interrupt_called : Error<
+  "interrupt service routine cannot be called directly">;
 def warn_interrupt_attribute_invalid : Warning<
    "%select{MIPS|MSP430|RISC-V}0 'interrupt' attribute only applies to "
    "functions that have %select{no parameters|a 'void' return type}1">,
diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
index 91ddad3e83ca5..c989b1dcc802b 100644
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -6755,9 +6755,15 @@ ExprResult Sema::BuildResolvedCallExpr(Expr *Fn, NamedDecl *NDecl,
   unsigned BuiltinID = (FDecl ? FDecl->getBuiltinID() : 0);
 
   // Functions with 'interrupt' attribute cannot be called directly.
-  if (FDecl && FDecl->hasAttr<AnyX86InterruptAttr>()) {
-    Diag(Fn->getExprLoc(), diag::err_anyx86_interrupt_called);
-    return ExprError();
+  if (FDecl) {
+    if (FDecl->hasAttr<AnyX86InterruptAttr>()) {
+      Diag(Fn->getExprLoc(), diag::err_anyx86_interrupt_called);
+      return ExprError();
+    }
+    if (FDecl->hasAttr<ARMInterruptAttr>()) {
+      Diag(Fn->getExprLoc(), diag::err_arm_interrupt_called);
+      return ExprError();
+    }
   }
 
   // X86 interrupt handlers may only call routines with attribute