[LLVMbugs] [Bug 23384] New: LSR needs to be improved for x86

[bugzilla-daemon at llvm.org]

https://llvm.org/bugs/show_bug.cgi?id=23384

            Bug ID: 23384
           Summary: LSR needs to be improved for x86
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: wmi at google.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Created attachment 14272
  --> https://llvm.org/bugs/attachment.cgi?id=14272&action=edit
testcase 1.cc

For the testcase attached, llvm now generates 3 AddRec but 1 AddRec is enough
by using complex addressing modes. Every AddRec will be translated to an add
insn at the end of loop, so the number of AddRec is important for loop
performance. And it is more important than NumRegs especially when register
pressure is not very high.

~/workarea/llvm-r234391/build/bin/clang++ -std=c++11 -O2
-fno-omit-frame-pointer -S 1.cc -o 1.s

The kernel loop:
.LBB1_10:                               # %for.body6.i
        movss   -4(%rcx), %xmm2         # xmm2 = mem[0],zero,zero,zero
        movss   (%rcx), %xmm1           # xmm1 = mem[0],zero,zero,zero
        mulss   -4(%rdi), %xmm2
        addss   %xmm0, %xmm2
        mulss   (%rdi), %xmm1
        addss   %xmm2, %xmm1
        addq    $8, %rdi
        addq    $8, %rcx
        addl    $-2, %esi
        movaps  %xmm1, %xmm0
        jne     .LBB1_10 

A better version is:
.LBB1_8:                                # %for.body6.i
        movss   (%rdi,%rcx,4), %xmm0    # xmm0 = mem[0],zero,zero,zero
        mulss   (%rsi,%rcx,4), %xmm0
        addss   %xmm1, %xmm0
        movss   4(%rdi,%rcx,4), %xmm1   # xmm1 = mem[0],zero,zero,zero
        mulss   4(%rsi,%rcx,4), %xmm1
        addss   %xmm0, %xmm1
        addq    $2, %rcx
        cmpl    %ecx, %eax
        movaps  %xmm1, %xmm0
        jne     .LBB1_8

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