aparna kotha
2011-May-25 18:09 UTC
[LLVMdev] Floating Point Register Allocation in X86 backend
Hi Guys, I was working on some floating point intensive benchmarks and realize that the floating point register allocation in llvm assumes that there are only 7 floating point registers in X86, whereas the hardware has 8. Line number 00266 assert(Reg >= X86::FP0 && Reg <= X86::FP6 && "Expected FP register!"); of X86FloatingPoint.cpp. Is there any reason for only counting from 0 to 6, when there are actually 8 in hardware ? Is there an assumption somewhere else, that I am missing. Thanks and Regards Aparna Kotha Graduate Student University of Maryland, College Park -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20110525/8879bee2/attachment.html>
Jakob Stoklund Olesen
2011-May-25 18:28 UTC
[LLVMdev] Floating Point Register Allocation in X86 backend
On May 25, 2011, at 11:09 AM, aparna kotha wrote:> Hi Guys, > > I was working on some floating point intensive benchmarks and realize that the floating point register allocation in llvm assumes that there are only 7 floating point registers in X86, whereas the hardware has 8. > > Line number > 00266 assert(Reg >= X86::FP0 && Reg <= X86::FP6 && "Expected FP register!"); > > of X86FloatingPoint.cpp. > > Is there any reason for only counting from 0 to 6, when there are actually 8 in hardware ?It has to do with the weird tricks that are needed to generate code for a stack machine.> Is there an assumption somewhere else, that I am missing.Yes, the default cpu on Linux is i386 which doesn't have SSE support. Use SSE if you care about floating point performance. I think -mcpu=... is all you need. /jakob
aparna kotha
2011-May-25 19:08 UTC
[LLVMdev] Floating Point Register Allocation in X86 backend
Right. But there are 8 registers on the floating point stack from ST0 to ST7 and I think llvm is only using ST0 to ST6 in some code fragments. Could this be because of the assumption that X86::FP registers run from X86::FP0 to X86:FP6 ? --Aparna On Wed, May 25, 2011 at 2:28 PM, Jakob Stoklund Olesen <stoklund at 2pi.dk>wrote:> > On May 25, 2011, at 11:09 AM, aparna kotha wrote: > > > Hi Guys, > > > > I was working on some floating point intensive benchmarks and realize > that the floating point register allocation in llvm assumes that there are > only 7 floating point registers in X86, whereas the hardware has 8. > > > > Line number > > 00266 assert(Reg >= X86::FP0 && Reg <= X86::FP6 && "Expected FP > register!"); > > > > of X86FloatingPoint.cpp. > > > > Is there any reason for only counting from 0 to 6, when there are > actually 8 in hardware ? > > It has to do with the weird tricks that are needed to generate code for a > stack machine. > > > Is there an assumption somewhere else, that I am missing. > > Yes, the default cpu on Linux is i386 which doesn't have SSE support. > > Use SSE if you care about floating point performance. I think -mcpu=... is > all you need. > > /jakob > >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20110525/553e635d/attachment.html>
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