<html>
<head>
<base href="https://bugs.llvm.org/">
</head>
<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [feature request] -fbuiltin-*"
href="https://bugs.llvm.org/show_bug.cgi?id=47280">47280</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>[feature request] -fbuiltin-*
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>unspecified
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>All
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>C
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>ndesaulniers@google.com
</td>
</tr>
<tr>
<th>CC</th>
<td>blitzrakete@gmail.com, clement.courbet@gmail.com, david.bolvansky@gmail.com, dgregor@apple.com, efriedma@quicinc.com, erik.pilkington@gmail.com, hpa@zytor.com, llvm-bugs@lists.llvm.org, richard-llvm@metafoo.co.uk
</td>
</tr></table>
<p>
<div>
<pre>From this thread on LKML:
<a href="https://lore.kernel.org/lkml/76071c24-ec6f-7f7a-4172-082bd574d581@zytor.com/">https://lore.kernel.org/lkml/76071c24-ec6f-7f7a-4172-082bd574d581@zytor.com/</a>
It was noted that for hosted environments that don't quite have a full libc
implementation, that they can lean on -fno-builtin-* flags in Clang to avoid
libcall optimizations where LLVM would emit calls to * which the environment
did not provide definitions of (though this differs from GCC's behavior, see
these three posts from Arvind that are a really fantastic analysis:
1. <a href="https://lore.kernel.org/lkml/20200818214146.GA3196105@rani.riverdale.lan/">https://lore.kernel.org/lkml/20200818214146.GA3196105@rani.riverdale.lan/</a>
2. <a href="https://lore.kernel.org/lkml/20200820175617.GA604994@rani.riverdale.lan/">https://lore.kernel.org/lkml/20200820175617.GA604994@rani.riverdale.lan/</a>
3. <a href="https://lore.kernel.org/lkml/20200821172935.GA1411923@rani.riverdale.lan/">https://lore.kernel.org/lkml/20200821172935.GA1411923@rani.riverdale.lan/</a>
).
Kernel developer H. Peter Anvin points out it would be helpful for embedded
environments that would use -ffreestanding to opt out of all such libcall
optimizations, to be able to opt back in (rather than opting out via
-fno-builtin-*) to certain libcall optimizations when the otherwise
freestanding implementation provided such symbols with matching semantics (ie.
the Linux kernel).
It was suggested that listing supported functions in the environment via
#pragma's (maybe in a header for codebase-wide inclusion) might be helpful. In
particular, I think it would slot in nicely in Clang to provide -fbuiltin-*
support (there's rules in tablegen to generate support for -f-* and -fno-* from
one rule). So `-ffreestanding -fbuiltin-strcpy` would be "disable all libcall
optimizations except related to transforming other libcalls into strcpy,
denoting the target supported no other builtins than strcpy (and the always
required mem* family).
It's not an immediate need, more so a curiosity that it's easy to opt out of
libcall optimizations, but not necessarily opt in.</pre>
</div>
</p>
<hr>
<span>You are receiving this mail because:</span>
<ul>
<li>You are on the CC list for the bug.</li>
</ul>
</body>
</html>