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[Bug target/42172] New: inefficient bit fields assignments


The attached test case contains several continuous bit fields assignment,
compile it with options -mthumb -Os, gcc generates

        .fnstart
.LFB1:
        ldrb    r3, [r0]
        mov     r2, #7
        bic     r3, r3, r2
        strb    r3, [r0]
        ldrb    r3, [r0]
        mov     r2, #56
        bic     r3, r3, r2
        mov     r2, #8
        orr     r3, r3, r2
        strb    r3, [r0]
        ldrb    r3, [r0]
        mov     r2, #64
        bic     r3, r3, r2
        strb    r3, [r0]
        ldrb    r2, [r0]
        mov     r3, #127
        and     r3, r3, r2
        strb    r3, [r0]
        @ sp needed for prologue
        bx      lr

The 4 fields are contained in one word, for each field assignment the code
loads the word, changes the field, then write the word back. A better code
sequence should load the word once, change all 4 fields, then write back the
changed word.

        ldrb    r3, [r0]
        mov     r2, #255             // bit mask
        bic     r3, r3, r2
        mov     r2, #8               // the new value of all 4 fields
        orr     r3, r3, r2
        strb    r3, [r0]
        @ sp needed for prologue
        bx      lr

or more aggressively if the word contains only these four fields

        mov     r3, #8
        strb    r3, [r0]
        @ sp needed for prologue
        bx      lr


-- 
           Summary: inefficient bit fields assignments
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: carrot at google dot com
 GCC build triplet: i686-linux
  GCC host triplet: i686-linux
GCC target triplet: arm-eabi


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42172


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