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Re: [Pocl-devel] Non-optimal code generation
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From: Erik S. <esc...@pe...> - 2011-10-30 22:21:53
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Pekka You are right; andss does not exist, but there is an andps instead. -erik 2011/10/30 Pekka Jääskeläinen <pek...@tu...>: > Hi Erik, > > On 10/29/2011 07:43 PM, Erik Schnetter wrote: >> When I use clang 3.1 (a recent snapshot) to translate e.g. the fabs >> intrinsic, acting on a single floating point number, then then >> generated x86 code looks like >> >> _Z4fabsf: # @_Z4fabsf >> movd %xmm0, %eax >> andl $2147483647, %eax # imm = 0x7FFFFFFF >> movd %eax, %xmm0 >> ret >> >> This is not optimal, since the value is moved from xmm0 to eax and >> back, which is not necessary. Instead of andl, I expect to see the >> andss instruction. >> >> How do I go about having this corrected? Is this a problem in pocl, in >> clang, in llvm, or in the way one of these are used? > > I'm not familiar with the SSE instruction extensions but quick googling > didn't return 'andss' for single floats. E.g.: > http://en.wikipedia.org/wiki/X86_instruction_listings > > I see this absf implementation uses bit manipulation to reset the sign bit > of the float word to return the absolute. Thus, in case SSE does not have > 'and', it has to go back to the x86 instruction set to perform the and > to reset the sign bit. > > If SSE has a suitable 'and', it should be able to operate directly on the > xmm reg in which case it's an LLVM instruction selection issue. In that case > overriding the implementation with inline assembly can circumvent the issue. > Of course, the preferred way is to add proper 'andss' to the instruction > patterns in the LLVM side, if such is available. > > -- > --Pekka > > > ------------------------------------------------------------------------------ > Get your Android app more play: Bring it to the BlackBerry PlayBook > in minutes. BlackBerry App World™ now supports Android™ Apps > for the BlackBerry® PlayBook™. Discover just how easy and simple > it is! http://p.sf.net/sfu/android-dev2dev > _______________________________________________ > Pocl-devel mailing list > Poc...@li... > https://lists.sourceforge.net/lists/listinfo/pocl-devel > -- Erik Schnetter <esc...@pe...> http://www.cct.lsu.edu/~eschnett/ AIM: eschnett247, Skype: eschnett, Google Talk: sch...@gm... |