[Bug target/40697] New: inefficient code to extract least bits from an integer value

["carrot at google dot com" <gcc-bugzilla at gcc dot gnu dot org>]

Compile following function with options -Os -mthumb -march=armv5te

unsigned get_least_bits(unsigned value)
{
  return value << 9 >> 9;
}

Gcc generates:

        ldr     r3, .L2
        @ sp needed for prologue
        and     r0, r0, r3
        bx      lr
.L3:
        .align  2
.L2:
        .word   8388607

A better code sequence should be:

       lsl   r0, 9
       lsr   r0, 9
       bx    lr

It is smaller (without constant pool) and faster.

This transformation was done very early and we can see it in the first tree
dump shift.c.003t.original. Gcc thinks and with a constant is cheaper than two
shifts. It is not true for this case in thumb ISA. On the other hand if the
constant used to and is small, such as 7, it is definitely cheaper than two
shifts. So which method is better is highly depend on both the constant and the
target ISA. It is difficult to make a correct decision in the TREE level. Maybe
we should define a peephole rule to do it.


-- 
           Summary: inefficient code to extract least bits from an integer
                    value
           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=40697