[LLVMbugs] [Bug 21600] New: [AArch64] Prefer tbz/tbnz before cmp+br in AND expression

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http://llvm.org/bugs/show_bug.cgi?id=21600

            Bug ID: 21600
           Summary: [AArch64] Prefer tbz/tbnz before cmp+br in AND
                    expression
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: ASSIGNED
          Severity: normal
          Priority: P
         Component: Backend: AArch64
          Assignee: mcrosier at codeaurora.org
          Reporter: mcrosier at codeaurora.org
                CC: kevinqindev at gmail.com, llvmbugs at cs.uiuc.edu,
                    t.p.northover at gmail.com
    Classification: Unclassified

Given the following IR,

  %cmp0 = icmp slt i64 %a, 0
  %cmp1 = icmp eq i32 %b, 1
  %and = and i1 %cmp0, %cmp1
  br i1 %and, label %if.then, label %if.end

the following assembly is generated:

        tbz     x0, #63, .LBB0_3
        cmp      w1, #1
        b.ne    .LBB0_3
        bl      t
.LBB0_3:                                // %if.end

However, if we commute the 'and' operands as such,

  %cmp0 = icmp slt i64 %a, 0
  %cmp1 = icmp eq i32 %b, 1
  %and = and i1 %cmp1, %cmp0
  br i1 %and, label %if.then, label %if.end

the following assembly is generated:

        cmp      w1, #1
        b.ne    .LBB1_3
        tbz     x0, #63, .LBB1_3
        bl      t
.LBB1_3:                                // %if.end

We should prefer the former code sequence by commuting the 'and' operands.

Complete test cases:
----------------------------------------------------------------------------
; RUN: llc -O1 -march=aarch64 < %s | FileCheck %s

declare void @t()

define void @test1(i64 %a, i32 %b) {
entry:
  %cmp0 = icmp slt i64 %a, 0
  %cmp1 = icmp eq i32 %b, 1
  %and = and i1 %cmp0, %cmp1
  br i1 %and, label %if.then, label %if.end

if.then:
  call void @t()
  br label %if.end

if.end:
  ret void
}

define void @test2(i64 %a, i32 %b) {
entry:
  %cmp0 = icmp slt i64 %a, 0
  %cmp1 = icmp eq i32 %b, 1
  %and = and i1 %cmp1, %cmp0
  br i1 %and, label %if.then, label %if.end

if.then:
  call void @t()
  br label %if.end

if.end:
  ret void
}

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