[LLVMbugs] [Bug 5313] New: Optimisation "loop-rotate" generate unoptimal dirty code

[bugzilla-daemon at cs.uiuc.edu]

http://llvm.org/bugs/show_bug.cgi?id=5313

           Summary: Optimisation "loop-rotate" generate unoptimal dirty code
           Product: new-bugs
           Version: trunk
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Keywords: code-quality
          Severity: minor
          Priority: P2
         Component: new bugs
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: quickslyver at free.fr
                CC: llvmbugs at cs.uiuc.edu


with this simple function:

$cat test-opt.c
void append_text(const char* text,unsigned char * const  io)
{
  while(*text)
    *io=*text++;
}

-------------------------------------
optimized with loop-rotate:

$clang-cc test-opt.c -O3 -o - -S -triple=thumb-unknown-unknown
(...)
append_text:
        ldrb r2, [r0]
        cmp r2, #0
        beq .LBB1_3
        movs r2, #0
.LBB1_2:
        ldrb r3, [r0, r2]
        strb r3, [r1]
        adds r2, r2, #1
        ldrb r3, [r0, r2]
        cmp r3, #0
        bne .LBB1_2
.LBB1_3:
        bx lr
        .size append_text, .-append_text

-----------------------------------------
Without loop-rotate:
$clang-cc test-opt.c -O0 -o - -emit-llvm-bc -triple=thumb-unknown-unknown | opt
-simplifycfg -domtree -domfrontier -scalarrepl -instcombine -simplifycfg
-globalopt -globaldce -ipconstprop -deadargelim -instcombine -simplifycfg
-basiccg -prune-eh -functionattrs -inline -argpromotion -simplify-libcalls
-instcombine -jump-threading -simplifycfg -domtree -domfrontier -scalarrepl
-instcombine -break-crit-edges -condprop -tailcallelim -simplifycfg
-reassociate -domtree -loops -loopsimplify -domfrontier -loopsimplify -lcssa 
-licm -lcssa -loop-unswitch -instcombine -scalar-evolution -loopsimplify -lcssa
-iv-users -loop-deletion -loopsimplify -lcssa -loop-unroll -instcombine -memdep
-gvn -memdep -memcpyopt -sccp -instcombine -break-crit-edges -condprop -domtree
-memdep -dse -adce -simplifycfg -strip-dead-prototypes -print-used-types
-deadtypeelim -constmerge -preverify -domtree -verify | llc -O3
-asm-verbose=false

(...)
.LBB1_1:
        ldrb r2, [r0]
        cmp r2, #0
        beq .LBB1_3
        strb r2, [r1]
        adds r0, r0, #1
        b .LBB1_1
.LBB1_3:
        bx lr

--------------------------------------
I target Thumb architecture because I read it easily, but you can directly dump
llvm-ir or others target: It is a problem at llvm-ir optimization stage.


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