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</o:shapelayout></xml><![endif]--></head><body lang=EN-GB link=blue vlink=purple><div class=WordSection1><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Forcing tailcalls, even when it isn’t profitable in terms of performance (or space in the case of<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>-Os, though I can’t off the top of my head think of any case where a faster tail call would also<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>be larger), is what the -tailcallopt option is for: see<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><a href="http://llvm.org/docs/CodeGenerator.html#tail-call-optimization">http://llvm.org/docs/CodeGenerator.html#tail-call-optimization</a><o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>John<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><div style='border:none;border-left:solid blue 1.5pt;padding:0cm 0cm 0cm 4.0pt'><div><div style='border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0cm 0cm 0cm'><p class=MsoNormal><b><span lang=EN-US style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>From:</span></b><span lang=EN-US style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'> bruce.hoult@gmail.com [mailto:bruce.hoult@gmail.com] <b>On Behalf Of </b>Bruce Hoult<br><b>Sent:</b> 13 January 2015 00:01<br><b>To:</b> John Brawn<br><b>Subject:</b> Re: [LLVMdev] [RFC] [PATCH] add tail call optimization to thumb1-only targets<o:p></o:p></span></p></div></div><p class=MsoNormal><o:p> </o:p></p><div><div><div><p class=MsoNormal>On Tue, Jan 13, 2015 at 3:50 AM, John Brawn <<a href="mailto:john.brawn@arm.com" target="_blank">john.brawn@arm.com</a>> wrote:<o:p></o:p></p><p class=MsoNormal>> During epilog generation, spill register restoring will be done within<br>> the emit epilogue function.<br>> If LR happens to be spilled on the stack by the prologue, it's restored<br>> by use of a scratch register<br>> just before restoring the other registers.<br><br>POP is 1+N cycles whereas LDR is 2 cycles. If we need to LDR lr from the<br>stack then POP r4 then that's 2 (LDR) + 1+1 (POP) + 1 (MOV to lr) + 1<br>(ADD sp) = 6 cycles, but a POP {r4,lr} is just 3 cycles.<o:p></o:p></p><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>You appear to be using speed as the figure of merit, but that is not the point of tail call optimisation (except incidentally).<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>TCO is to minimise the use of precious stack space, and in fact to allow certain algorithms and program transformations to run in constant stack space.<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>If a programmer assumes that TCO is available and writes their program using continuation-passing style, and then TCO does not actually happen, that is a correctness issue and the program will overflow the stack and crash very quickly.<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>A few register loads or spills is a distant second consideration.<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div></div></div></div></div></div></body></html>