<html><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><br><div><div>On Mar 30, 2008, at 10:21 AM, Chris Lattner wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><div><div>On Mar 28, 2008, at 5:17 PM, Chuck Rose III wrote:</div><blockquote type="cite"><span class="Apple-style-span" style="border-collapse: separate; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-align: auto; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0; "><div lang="EN-US" link="blue" vlink="purple"><div class="Section1"><div style="margin-top: 0in; margin-right: 0in; margin-left: 0in; margin-bottom: 0.0001pt; font-size: 12pt; font-family: 'Times New Roman'; "><font size="2" face="Arial"><span style="font-size: 10pt; font-family: Arial; ">I was curious about the state of stack alignment on x86. I noticed there are a few bugs outstanding on the issue. I recently added some code which had the effect of throwing an extra function parameter on our stack at runtime, a 4 byte pointer. <o:p></o:p></span></font></div><div style="margin-top: 0in; margin-right: 0in; margin-left: 0in; margin-bottom: 0.0001pt; font-size: 12pt; font-family: 'Times New Roman'; "><font size="2" face="Arial"><span style="font-size: 10pt; font-family: Arial; "><o:p> </o:p></span></font></div><div style="margin-top: 0in; margin-right: 0in; margin-left: 0in; margin-bottom: 0.0001pt; font-size: 12pt; font-family: 'Times New Roman'; "><font size="2" face="Arial"><span style="font-size: 10pt; font-family: Arial; ">Esp is now not 16-byte aligned, so instructions like unpcklps xmm1, dword ptr [eps] cause grief. My AllocaInstr instructions are told to be 16 byte aligned, so the addition of a 4-byte parameter shouldn’t have changed alignment on the objects.<o:p></o:p></span></font></div><div style="margin-top: 0in; margin-right: 0in; margin-left: 0in; margin-bottom: 0.0001pt; font-size: 12pt; font-family: 'Times New Roman'; "><font size="2" face="Arial"><span style="font-size: 10pt; font-family: Arial; "><o:p></o:p></span></font></div></div></div></span></blockquote><div><br class="webkit-block-placeholder"></div><div>Hi Chuck,</div><div><br class="webkit-block-placeholder"></div><div>I think the basic problem is that the stack pointer on windows/linux is not guaranteed to be 16 byte aligned. This means that any use of an instruction which requires 16-byte alignment (e.g. sse stuff) and accesses a frameindex can cause a problem. The issue is that the frameindex will be marked as needing 16+ byte alignment, but the code generator just won't respect this.</div><div><br class="webkit-block-placeholder"></div><div>The fix for this is somewhat simple: in Prolog/Epilog Insertion, the PEI pass should notice when frame indices have alignment greater than the guaranteed stack alignment. When this happens, it should emit code into the prolog to dynamically align the stack (e.g. by emitting 'and esp, -16').</div></div></div></blockquote><blockquote type="cite"><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><div><div><br></div></div></div></blockquote><blockquote type="cite"><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><div><div>This doesn't occur on the mac, because the stack is always guaranteed to be 16-byte aligned.</div></div></div></blockquote><br></div><div>Another possibility, which only works if you have control of all the code being compiled, is to 16-byte align the stack in main and keep it 16-byte aligned thereafter. Different versions of gcc have used this method and the method Chris suggests.</div><div><br class="webkit-block-placeholder"></div><div>Still another is to disable generation of SSE instructions. -mattr=-sse2 should work. The gcc switch, -mno-sse2, did not work in llvm-gcc last time I tried; this is on my list of things to make work, but fairly far down.</div><div><br></div></body></html>