Ok, <br><br>I'm trying to get llvm to decide when to turn the multiplier "on" and "off" for an ARM simulator. These instructions are just to let the simulator know when to power on/off the multiplier. I need to insert an instruction before the first multiply and another one after the last multiply in a basic block. I'm currently trying to figure out the best way to do that without having to add a new instruction to llvm. <br>
<br>My plan was to insert useless instructions, let llc generate an assembly (.s) file, then use sed to replace the useless instructions with my special instructions. However, llc eliminates my useless instructions. ( Allocas and binary operators go away, volatile loads and stores change the register allocation. )<br>
<br>%mul_en = alloca i32<br>%2 = mul nsw i32 %y, %x<br>%mul_dis = alloca i32<br><br>becomes...<br><br> mul r12, r12, r0<br><br><br>Next I tried using llvm-dis and llvm-as to manually replace my useless
instructions with a inline no-op in the .bc file. However, llc then
moves the volatile no-op below the multiply. <br>
<br>tail call void asm sideeffect "NOP", ""() nounwind<br>%2 = mul nsw i32 %y, %x<br>tail call void asm sideeffect "NOP", ""() nounwind<br><br clear="all">becomes....<br><br>mul r12, r12, r0<br>
...<br>@APP<br>NOP<br>@NO_APP<br>@APP<br>NOP<br>@NO_APP<br><br><br>Andrew Lukefahr<br><a href="mailto:andrewlukefahr@gmail.com" target="_blank">andrewlukefahr@gmail.com</a><br>
<br>
Open Source, Open Minds<br>
<br><br><div class="gmail_quote">On Mon, Dec 6, 2010 at 2:08 PM, 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: 0pt 0pt 0pt 0.8ex; border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;">
<div><br>
On Dec 6, 2010, at 10:57 AM, John Criswell wrote:<br>
<br>
> On 12/6/10 11:39 AM, Andrew Lukefahr wrote:<br>
>> Hi,<br>
>><br>
>> How would I disable dead code elimination in llc? Can that be done via the command line or do I need to modify llc's source?<br>
><br>
> You can use llc --help-hidden to see a list of options. There are several -disable-xxx options that disable various code generator optimizations.<br>
><br>
> That said, I, like Duncan, wasn't aware that llc did any dead-code elimination (beyond peephole optimization). Are you sure llc is removing the instructions you care about?<br>
><br>
<br>
<br>
</div>Perhaps the dead machine instruction elimination pass? (DeadMachineInstructionElim.cpp) I don't think there's a way to disable that.<br>
<font color="#888888"><br>
-Jim<br>
<br>
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