[cfe-commits] r149086 - in /cfe/trunk: include/clang/Basic/TargetInfo.h lib/Basic/Targets.cpp lib/CodeGen/CGBuiltin.cpp test/CodeGen/builtin-count-zeros.c

[Chandler Carruth]

On Thu, Jan 26, 2012 at 2:14 PM, Bob Wilson <bob.wilson at apple.com> wrote:

> Make clz/ctz builtins defined for zero on ARM targets.  rdar://10732455
>
> ARM supports clz and ctz directly and both operations have well-defined
> results for zero.  There is no disadvantage in performance to using the
> defined-at-zero versions of llvm.ctlz/cttz intrinsics.  We're running into
> ARM-specific code written with the assumption that __builtin_clz(0) == 32,
> even though that value is technically undefined.  The code is failing now
> because of llvm optimizations that are taking advantage of the undef
> behavior (specifically svn r147255).  There's nothing wrong with that
> optimization on x86 where any incorrect assumptions about __builtin_clz(0)
> will quickly be exposed.  For ARM, though, optimizations based on that
> undef
> behavior are likely to cause subtle bugs.  Other targets with
> defined-at-zero
> clz/ctz support may want to override the default behavior as well.
>

Ouch. I can't argue at all with the logic here, but this is a nasty, nasty
minefield.

I wonder, would it be better to add new builtins to represent
defined-at-zero semantics consistently on all platforms? Maybe we could get
folks to switch to those (or at least start).

Either way, please document this language extension really loudly, as it is
a specific divergence from GCC's specification of the builtins, and the
builtins are supposedly Clang providing a "faithful" emulation of GCC....
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