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<p>[+Richard, Chandler]<br>
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<div class="moz-cite-prefix">On 10/09/2017 07:00 PM, Hans Wennborg
via cfe-dev wrote:<br>
</div>
<blockquote
cite="mid:CAB8jPhdEO9MZW=BapwP0QQ5wi9Dr5MiVgFi9yUAMJkWay5+-JA@mail.gmail.com"
type="cite">
<pre wrap="">I am not a language lawyer, but I'll atttempt to answer anyway.
On Mon, Oct 9, 2017 at 2:16 PM, Kreitzer, David L via cfe-dev
<a class="moz-txt-link-rfc2396E" href="mailto:cfe-dev@lists.llvm.org"><cfe-dev@lists.llvm.org></a> wrote:
</pre>
<blockquote type="cite">
<pre wrap="">This llvm patch, <a class="moz-txt-link-freetext" href="https://reviews.llvm.org/D37289">https://reviews.llvm.org/D37289</a>, attempts to do an optimization
that involves speculating loads. The patch itself needs some work regardless,
but we are questioning the legality of the optimization, which is currently
performed by both gcc and icc. The desired transformation looks like this:
typedef struct S {
char padding[4088];
struct S *p1;
struct S *p2;
} S;
struct S* f1(struct S *s, int x)
{
S *r;
if (x)
r = s->p1;
else
r = s->p2;
return r;
}
TO
struct S* f1(struct S *s, int x)
{
return (x) ? s->p1 : s->p2;
}
The fundamental question seems to be whether loading one member of struct S
makes it valid to load other members of the same struct.
</pre>
</blockquote>
<pre wrap="">
Yes, I believe that's true. If we're dereferencing s on both paths, it
must point to a struct S object, and then loading from any member of
that object should be fine.</pre>
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<br>
I also believe that this is correct.<br>
<br>
I think that Chandler summarized some things to be careful about in
this regard here:<br>
<a class="moz-txt-link-freetext" href="http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20170807/477944.html">http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20170807/477944.html</a><br>
<br>
Of the three points highlighted here, the second clearly might
apply:<br>
<blockquote type="cite">
<meta http-equiv="content-type" content="text/html; charset=utf-8">
<pre style="white-space: pre-wrap; color: rgb(0, 0, 0); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px;">2) Related to #1, there are applications that rely on this memory model,
for example structures where entire regions of the structure live in
protected pages and cannot be correctly accessed.</pre>
</blockquote>
<br>
This, however, is clearly an extension to the standard memory model,
and I see no reason to support this by default. Speculatively
loading cache lines under contention from other cores might not be a
good thing to do for performance reasons, but that's not a
correctness issue.<br>
<br>
-Hal<br>
<br>
<blockquote
cite="mid:CAB8jPhdEO9MZW=BapwP0QQ5wi9Dr5MiVgFi9yUAMJkWay5+-JA@mail.gmail.com"
type="cite">
<pre wrap="">
</pre>
<blockquote type="cite">
<pre wrap="">Both gcc & icc
seem to think so and make a distinction between this case and a similar case
where one field is accessed through an lvalue of a different type:
typedef struct T {
char padding[4088];
struct S *p1;
} T;
struct S* f2(struct S *s, int x)
{
S *r;
if (x)
r = ((T*)s)->p1;
else
r = s->p2;
return r;
}
Neither compiler will transform this case.
</pre>
</blockquote>
<pre wrap="">
I suspect it would be within the compiler's rights, but my language
knowledge is too weak. Does the cast imply that s points to a valid
struct S object (or null, but then we couldn't dereference it)? I'm
curious to find out :-)
- Hans
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</pre>
</blockquote>
<br>
<pre class="moz-signature" cols="72">--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory</pre>
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