[llvm-bugs] [Bug 37789] New: [AArch64] machine combiner converts independent fadds into dependent fadds

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=37789

            Bug ID: 37789
           Summary: [AArch64] machine combiner converts independent fadds
                    into dependent fadds
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: AArch64
          Assignee: unassignedbugs at nondot.org
          Reporter: spatel+llvm at rotateright.com
                CC: llvm-bugs at lists.llvm.org

Copied from:
test/CodeGen/AArch64/fadd-combines.ll

; DAGCombiner transforms this into: (x + 59.0) + (x + 17.0).
; The machine combiner transforms this into a chain of 3 dependent adds:
; ((x + 59.0) + 17.0) + x

define float @fadd_const_multiuse_attr(float %x) #0 {
; CHECK-LABEL: fadd_const_multiuse_attr:
; CHECK:       // %bb.0:
; CHECK-NEXT:    adrp x9, .LCPI8_1
; CHECK-NEXT:    adrp x8, .LCPI8_0
; CHECK-NEXT:    ldr s1, [x9, :lo12:.LCPI8_1]
; CHECK-NEXT:    ldr s2, [x8, :lo12:.LCPI8_0]
; CHECK-NEXT:    fadd s1, s0, s1
; CHECK-NEXT:    fadd s1, s2, s1
; CHECK-NEXT:    fadd s0, s0, s1
; CHECK-NEXT:    ret
  %a1 = fadd float %x, 42.0
  %a2 = fadd float %a1, 17.0
  %a3 = fadd float %a1, %a2
  ret float %a3
}

---------------------------------------------------------------------------

That's with no CPU model specified. When I tried setting that to various
existing models, the transform doesn't happen. 

I don't see why we would do this transform under any circumstances, but I might
be missing some AArch subtlety.

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