host-utils: Drop workaround for buggy Apple Clang __builtin_subcll()

[PATCH] host-utils: Drop workaround for buggy Apple Clang __builtin_subcll()

Posted by Peter Maydell 4 months ago

In commit b0438861efe ("host-utils: Avoid using __builtin_subcll on
buggy versions of Apple Clang") we added a workaround for a bug in
Apple Clang 14 where its __builtin_subcll() implementation was wrong.
This bug was only present in Apple Clang 14, not in upstream clang,
and is not present in Apple Clang versions 15 and newer.

Since commit 4e035201 we have required at least Apple Clang 15, so we
no longer build with the buggy versions.  We can therefore drop the
workaround. This is effectively a revert of b0438861efe.

This should not be backported to stable branches, which may still
need to support Apple Clang 14.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/3030
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
 include/qemu/compiler.h   | 13 -------------
 include/qemu/host-utils.h |  2 +-
 2 files changed, 1 insertion(+), 14 deletions(-)

diff --git a/include/qemu/compiler.h b/include/qemu/compiler.h
index 65b89958d36..1c2b673c058 100644
--- a/include/qemu/compiler.h
+++ b/include/qemu/compiler.h
@@ -182,19 +182,6 @@
 #define QEMU_DISABLE_CFI
 #endif
 
-/*
- * Apple clang version 14 has a bug in its __builtin_subcll(); define
- * BUILTIN_SUBCLL_BROKEN for the offending versions so we can avoid it.
- * When a version of Apple clang which has this bug fixed is released
- * we can add an upper bound to this check.
- * See https://gitlab.com/qemu-project/qemu/-/issues/1631
- * and https://gitlab.com/qemu-project/qemu/-/issues/1659 for details.
- * The bug never made it into any upstream LLVM releases, only Apple ones.
- */
-#if defined(__apple_build_version__) && __clang_major__ >= 14
-#define BUILTIN_SUBCLL_BROKEN
-#endif
-
 #if __has_attribute(annotate)
 #define QEMU_ANNOTATE(x) __attribute__((annotate(x)))
 #else
diff --git a/include/qemu/host-utils.h b/include/qemu/host-utils.h
index 4d28fa22cfa..dd558589cb5 100644
--- a/include/qemu/host-utils.h
+++ b/include/qemu/host-utils.h
@@ -677,7 +677,7 @@ static inline uint64_t uadd64_carry(uint64_t x, uint64_t y, bool *pcarry)
  */
 static inline uint64_t usub64_borrow(uint64_t x, uint64_t y, bool *pborrow)
 {
-#if __has_builtin(__builtin_subcll) && !defined(BUILTIN_SUBCLL_BROKEN)
+#if __has_builtin(__builtin_subcll)
     unsigned long long b = *pborrow;
     x = __builtin_subcll(x, y, b, &b);
     *pborrow = b & 1;
-- 
2.43.0

Re: [PATCH] host-utils: Drop workaround for buggy Apple Clang __builtin_subcll()

Posted by Philippe Mathieu-Daudé 4 months ago

On 14/7/25 16:50, Peter Maydell wrote:
> In commit b0438861efe ("host-utils: Avoid using __builtin_subcll on
> buggy versions of Apple Clang") we added a workaround for a bug in
> Apple Clang 14 where its __builtin_subcll() implementation was wrong.
> This bug was only present in Apple Clang 14, not in upstream clang,
> and is not present in Apple Clang versions 15 and newer.
> 
> Since commit 4e035201 we have required at least Apple Clang 15, so we
> no longer build with the buggy versions.  We can therefore drop the
> workaround. This is effectively a revert of b0438861efe.
> 
> This should not be backported to stable branches, which may still
> need to support Apple Clang 14.
> 
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/3030
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> ---
>   include/qemu/compiler.h   | 13 -------------
>   include/qemu/host-utils.h |  2 +-
>   2 files changed, 1 insertion(+), 14 deletions(-)

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>

Re: [PATCH] host-utils: Drop workaround for buggy Apple Clang __builtin_subcll()

Posted by Peter Maydell 4 months ago

On Mon, 14 Jul 2025 at 20:52, Philippe Mathieu-Daudé <philmd@linaro.org> wrote:
>
> On 14/7/25 16:50, Peter Maydell wrote:
> > In commit b0438861efe ("host-utils: Avoid using __builtin_subcll on
> > buggy versions of Apple Clang") we added a workaround for a bug in
> > Apple Clang 14 where its __builtin_subcll() implementation was wrong.
> > This bug was only present in Apple Clang 14, not in upstream clang,
> > and is not present in Apple Clang versions 15 and newer.
> >
> > Since commit 4e035201 we have required at least Apple Clang 15, so we
> > no longer build with the buggy versions.  We can therefore drop the
> > workaround. This is effectively a revert of b0438861efe.
> >
> > This should not be backported to stable branches, which may still
> > need to support Apple Clang 14.
> >
> > Resolves: https://gitlab.com/qemu-project/qemu/-/issues/3030
> > Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> > ---
> >   include/qemu/compiler.h   | 13 -------------
> >   include/qemu/host-utils.h |  2 +-
> >   2 files changed, 1 insertion(+), 14 deletions(-)
>
> Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>

Thanks all for the review; I'll pick this up via target-arm.next.

-- PMM

Re: [PATCH] host-utils: Drop workaround for buggy Apple Clang __builtin_subcll()

Posted by Richard Henderson 4 months ago

On 7/14/25 08:50, Peter Maydell wrote:
> In commit b0438861efe ("host-utils: Avoid using __builtin_subcll on
> buggy versions of Apple Clang") we added a workaround for a bug in
> Apple Clang 14 where its __builtin_subcll() implementation was wrong.
> This bug was only present in Apple Clang 14, not in upstream clang,
> and is not present in Apple Clang versions 15 and newer.
> 
> Since commit 4e035201 we have required at least Apple Clang 15, so we
> no longer build with the buggy versions.  We can therefore drop the
> workaround. This is effectively a revert of b0438861efe.
> 
> This should not be backported to stable branches, which may still
> need to support Apple Clang 14.
> 
> Resolves:https://gitlab.com/qemu-project/qemu/-/issues/3030
> Signed-off-by: Peter Maydell<peter.maydell@linaro.org>
> ---
>   include/qemu/compiler.h   | 13 -------------
>   include/qemu/host-utils.h |  2 +-
>   2 files changed, 1 insertion(+), 14 deletions(-)

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

Commit 4e035201 was in 9.2.0, but indeed there's no point in backporting to any stable branch.


r~

Re: [PATCH] host-utils: Drop workaround for buggy Apple Clang __builtin_subcll()

Posted by Thomas Huth 4 months ago

On 14/07/2025 16.50, Peter Maydell wrote:
> In commit b0438861efe ("host-utils: Avoid using __builtin_subcll on
> buggy versions of Apple Clang") we added a workaround for a bug in
> Apple Clang 14 where its __builtin_subcll() implementation was wrong.
> This bug was only present in Apple Clang 14, not in upstream clang,
> and is not present in Apple Clang versions 15 and newer.
> 
> Since commit 4e035201 we have required at least Apple Clang 15, so we
> no longer build with the buggy versions.  We can therefore drop the
> workaround. This is effectively a revert of b0438861efe.
> 
> This should not be backported to stable branches, which may still
> need to support Apple Clang 14.
> 
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/3030
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> ---
>   include/qemu/compiler.h   | 13 -------------
>   include/qemu/host-utils.h |  2 +-
>   2 files changed, 1 insertion(+), 14 deletions(-)

Reviewed-by: Thomas Huth <thuth@redhat.com>