<html><head><meta http-equiv="Content-Type" content="text/html charset=iso-8859-1"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><br><div><div>On Feb 5, 2013, at 11:03 AM, Eric Christopher <<a href="mailto:echristo@gmail.com">echristo@gmail.com</a>> wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><div dir="ltr">Well, I think we've gone from a compile time error for code like this:<div><br></div><div style="">void foo () {</div><div style=""> my_32byte_align_requested_struct a;</div><div style=""><br></div><div style=""> <insert inline asm that depends upon 32-byte alignment></div>
<div style=""><br></div><div style="">}</div><div style=""><br></div><div style="">to a run time crash.</div></div></blockquote><div><br></div>That is true, with this patch, we are going from a compile time error with r172027 to a run time crash with warning, for the above case.</div><div>If the source code does not depend on the 32-byte alignment, we are going from a compile time error to a warning.</div><div><br></div><div>Before r<span style="font-size: 12px;">172027, it is a run time crash without any warning.</span></div><div><span style="font-size: 12px;"><br></span></div><div><span style="font-size: 12px;">Let me know what you think :]</span></div><div><span style="font-size: 12px;"><br></span></div><div><span style="font-size: 12px;">Thanks,</span></div><div><span style="font-size: 12px;">Manman</span></div><div><span style="font-size: 12px;"> </span><br><blockquote type="cite"><div dir="ltr"><div style=""><br></div><div style="">It may be pretty uncommon, but perhaps some other examples for the type of code you're seeing would be good? I remember some code with 4 byte alignment depending upon 16-byte alignment at one point being difficult to track down due to this...</div>
<div style=""><br></div><div style="">-eric</div></div><div class="gmail_extra"><br><br><div class="gmail_quote">On Tue, Feb 5, 2013 at 10:53 AM, Jim Grosbach <span dir="ltr"><<a href="mailto:grosbach@apple.com" target="_blank">grosbach@apple.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin: 0px 0px 0px 0.8ex; border-left-width: 1px; border-left-color: rgb(204, 204, 204); border-left-style: solid; padding-left: 1ex; position: static; z-index: auto;"><div style="word-wrap:break-word">Thanks for clarifying.<div><br></div><div>With that in mind, I'm not personally opposed to demoting this to a warning anyway, but I will grant that it does somewhat weaken the case for it.</div>
<div><br></div><div>Eric, what do you think?</div><span class="HOEnZb"><font color="#888888"><div><br></div><div>-Jim</div></font></span><div><div class="h5"><br><div><div>On Feb 5, 2013, at 10:43 AM, Manman Ren <<a href="mailto:mren@apple.com" target="_blank">mren@apple.com</a>> wrote:</div>
<br><blockquote type="cite"><div style="word-wrap:break-word"><div><br></div>Sorry for the confusion :]<div>We are not performing the frontend analysis to check for whether the low bits are assumed to be zeros.</div><div>
I was stating that we should emit a hard error if the low bits are assumed to be zeros in the source code, and a warning if the low bits are not assumed to be zeros.</div><div>But since we don't currently perform the frontend analysis, to make sure existing codes that compile with previous version can still build, we emit a warning.</div>
<div><br></div><div>Hope that clears things out.</div><div><br></div><div>Manman</div><div><br><div><div>On Feb 5, 2013, at 10:36 AM, Jim Grosbach <<a href="mailto:grosbach@apple.com" target="_blank">grosbach@apple.com</a>> wrote:</div>
<br><blockquote type="cite"><div style="word-wrap:break-word">Hi Manman,<div><br></div><div><blockquote type="cite">Per discussion in <a>rdar://13127907</a>, we should emit a hard error only if<br>people write code where the requested alignment is larger than achievable<br>
and assumes the low bits are zeros. A warning should be good enough when<br>we are not sure if the source code assumes the low bits are zeros.</blockquote><div><br></div><div>I'm a bit confused. This implies that we're doing the frontend analysis to check for that condition and issue a hard error for it. The below is saying that's not actually the case. Can you elaborate a bit on what exactly is happening?</div>
</div></div></blockquote><blockquote type="cite"><div style="word-wrap:break-word"><div><br></div><div>-Jim</div><div><br><div><div>On Feb 5, 2013, at 10:18 AM, Manman Ren <<a href="mailto:mren@apple.com" target="_blank">mren@apple.com</a>> wrote:</div>
<br><blockquote type="cite"><div style="word-wrap:break-word"><div><br></div>We currently do not analyze the source code to check the usage of the low bits.<div><br><div>In the backend, the alignment is already clamped to the correct value, so the backend optimizations will not treat those low bits as zero.</div>
<div><br></div><div>-Manman</div><div><br></div><div><div><div>On Feb 4, 2013, at 6:08 PM, Eric Christopher <<a href="mailto:echristo@gmail.com" target="_blank">echristo@gmail.com</a>> wrote:</div><br><blockquote type="cite">
<div dir="ltr">Also I could be missing it but I couldn't spot the code that checks for the usage of the bits in not aligning/warning/erroring? Quick pointer?<div><br></div><div>Thanks!</div><div><br></div><div>
-eric</div></div><div class="gmail_extra"><br><br><div class="gmail_quote">On Mon, Feb 4, 2013 at 5:41 PM, Eric Christopher <span dir="ltr"><<a href="mailto:echristo@gmail.com" target="_blank">echristo@gmail.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><br><div class="gmail_extra"><br><br><div class="gmail_quote"><div>On Mon, Feb 4, 2013 at 5:35 PM, Manman Ren <span dir="ltr"><<a href="mailto:mren@apple.com" target="_blank">mren@apple.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word"><div><br></div>Yes, there are related discussions in r169197 and "[PATCH] Stack Alignment: clamp the alignment of stack objects in MachineFrameInfo".<div>
<br></div><div>But people can use a 32-byte alignment attribute on a machine which only supports 16-byte stack alignment.</div><div>If the source code further assumes the low bits are zeros, they will get wrong result.</div>
<div>But if not, a hard error is too much and it will make existing code which can compile with earlier version failed to build with this patch.</div></div></blockquote><div><br></div></div><div>And to use the other side of the argument that won last time :)</div>
<div><br></div><div>But this means that if people aren't looking at the warning or hard erroring on warnings then we're going to emit bad code instead of making it an error.</div><span><font color="#888888"><div>
<br></div><div>
-eric</div></font></span></div><br></div></div>
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