[Bug target/102135] New: (ARM Cortex-M3 and newer) changing operation order may reduce number of instructions needed

[Bug target/102135] New: (ARM Cortex-M3 and newer) changing operation order may reduce number of instructions needed

[jankowski938 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102135

            Bug ID: 102135
           Summary: (ARM Cortex-M3 and newer) changing operation order
                    may reduce number of instructions needed
           Product: gcc
           Version: 10.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jankowski938 at gmail dot com
  Target Milestone: ---

uint64_t foo64(const uint8_t *rData1)
{
    uint64_t buffer;
    buffer =  (((uint64_t)rData1[7]) << 56)|((uint64_t)(rData1[6]) <<
48)|((uint64_t)(rData1[5]) << 40)|(((uint64_t)rData1[4]) << 32)|
                            (((uint64_t)rData1[3]) <<
24)|(((uint64_t)rData1[2]) << 16)|((uint64_t)(rData1[1]) << 8)|rData1[0];

----
foo64:
        mov     r3, r0
        ldr     r0, [r0]  @ unaligned
        ldr     r1, [r3, #4]      @ unaligned
        bx      lr

Only 3 instructions are needed:

        ldr     r1, [r0, #4]      @ unaligned
        ldr     r0, [r0]  @ unaligned
        bx      lr

Options:
-O3 -mthumb -mcpu=cortex-M4

[Bug target/102135] (ARM Cortex-M3 and newer) changing operation order may reduce number of instructions needed

[rearnsha at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102135

--- Comment #1 from Richard Earnshaw <rearnsha at gcc dot gnu.org> ---
A small change to the testcase shows that this is highly dependent on the
constrained registers from the calling convention.  

uint64_t foo64(int dummy, const uint8_t *rData1)
{
    uint64_t buffer;
    buffer =  (((uint64_t)rData1[7]) << 56)|((uint64_t)(rData1[6]) <<
48)|((uint64_t)(rData1[5]) << 40)|(((uint64_t)rData1[4]) << 32)|
                            (((uint64_t)rData1[3]) <<
24)|(((uint64_t)rData1[2]) << 16)|((uint64_t)(rData1[1]) << 8)|rData1[0];

}

Register allocation does not re-order code in order to reduce the conflicts, so
this is not easy to fix.

This is also a problem that is more obvious in micro-testcases such as this
example, in real code it is more common for the register allocator to have more
freedom and to be able to avoid issues like this.  If your programming style is
to write functions like this you'd likely get better code overall by marking
these very small functions as inline, so that they do not incur the call setup
and call/return overhead, which can be significant when you take into account
the number of registers that must be saved over a function call.