Paul Eggert wrote:
> On 8/17/20 4:37 PM, Bruno Haible wrote:
> >> + Avoid Clang’s __builtin_assume, as clang 9.0.1 -Wassume can
> >> + generate a bogus diagnostic "the argument to '__builtin_assume' has
> >> + side effects that will be discarded" even when the argument has no
> >> + side effects. */
> > Do you have a test case, that we could check on clang 10 and on future
> > clang versions?
>
> Here's a short test that elicits the bogus warning for me now.
>
> static int f (int x) { return x; }
> int main (void) { __builtin_assume (f (1)); return 0; }
Thanks. I confirm it's still the same with clang 10.
But a small modification of the test case produces no warning:
static __attribute__ ((__const__)) int f (int x) { return x; }
int main (void) { __builtin_assume (f (1)); return 0; }
I find it quite natural that
* If you want to tell the compiler that it can make assumptions about
a function call, the compiler can evaluate the function call at
compile-time. If you don't want this, write
static int f (int x) { return x; }
int main (void) { int r = f (1); __builtin_assume (r); return 0; }
* You need to mark those functions that the compiler may evaluate
at compile-time.
* There is a diagnostic if the compiler can't take benefit of the
__builtin_assume invocation, although you intended it to have some.
So, I don't think the warning is bogus.
Back to the verify module. The warning tells us to move the side effect
outside of __builtin_assume. If I do this, I get an assume() macro that
* produces no warning,
* in clang versions < 9, has the desired optimization effect, whereas
the current definition doesn't.
Test case:
==================================================================
#if 1
/* Current definition */
# define assume(R) ((R) ? (void) 0 : __builtin_unreachable ())
#else
/* Proposed definition */
# define assume(R) \
((void) ({ __typeof__ (R) _gl_verify_temp = (R); \
__builtin_assume (_gl_verify_temp); }))
#endif
static int f (int x) { return x; }
int main (void) { assume (f (1)); return 0; }
int g (int x)
{
assume (x >= 4);
return (x > 1 ? x + 3 : 2 * x + 10);
}
==================================================================
With clang 8 and the current definition:
g: # @g
leal 3(%rdi), %ecx
cmpl $1, %edi
leal 10(%rdi,%rdi), %eax
cmovgl %ecx, %eax
retq
With clang 8 and the proposed definition:
g: # @g
leal 3(%rdi), %eax
retq
Here's a proposed patch.
2020-08-22 Bruno Haible <br...@clisp.org>
verify: Do use __built_assume on clang.
* lib/verify.h (assume): Use clang’s __builtin_assume, with a temporary
variable in a statement expression.
diff --git a/lib/verify.h b/lib/verify.h
index d485a02..0f3c6f9 100644
--- a/lib/verify.h
+++ b/lib/verify.h
@@ -246,6 +246,13 @@ template <int w>
/* @assert.h omit start@ */
+#if defined __has_builtin
+/* <https://clang.llvm.org/docs/LanguageExtensions.html#builtin-functions> */
+# define _GL_HAS_BUILTIN_ASSUME __has_builtin (__builtin_assume)
+#else
+# define _GL_HAS_BUILTIN_ASSUME 0
+#endif
+
#if 3 < __GNUC__ + (3 < __GNUC_MINOR__ + (4 <= __GNUC_PATCHLEVEL__))
# define _GL_HAS_BUILTIN_TRAP 1
#elif defined __has_builtin
@@ -305,14 +312,16 @@ template <int w>
Although assuming R can help a compiler generate better code or
diagnostics, performance can suffer if R uses hard-to-optimize
- features such as function calls not inlined by the compiler.
-
- Avoid Clang’s __builtin_assume, as clang 9.0.1 -Wassume can
- generate a bogus diagnostic "the argument to '__builtin_assume' has
- side effects that will be discarded" even when the argument has no
- side effects. */
-
-#if _GL_HAS_BUILTIN_UNREACHABLE
+ features such as function calls not inlined by the compiler. */
+
+#if _GL_HAS_BUILTIN_ASSUME
+/* Use a temporary variable, to avoid a clang warning
+ "the argument to '__builtin_assume' has side effects that will be discarded"
+ if R contains invocations of functions not marked as 'const'. */
+# define assume(R) \
+ ((void) ({ __typeof__ (R) _gl_verify_temp = (R); \
+ __builtin_assume (_gl_verify_temp); }))
+#elif _GL_HAS_BUILTIN_UNREACHABLE
# define assume(R) ((R) ? (void) 0 : __builtin_unreachable ())
#elif 1200 <= _MSC_VER
# define assume(R) __assume (R)
