[Bug rtl-optimization/90249] New: [9 regression] Code size regression on thumb2 due to sub-optimal register allocation.

[rearnsha at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90249

            Bug ID: 90249
           Summary: [9 regression] Code size regression on thumb2 due to
                    sub-optimal register allocation.
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Keywords: ra
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rearnsha at gcc dot gnu.org
                CC: ramana.radhakrishnan at arm dot com, vmakarov at redhat dot com,
                    wdijkstr at arm dot com
  Target Milestone: ---
            Target: arm

Created attachment 46244
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=46244&action=edit
testcase

GCC 9 has regressed on code size due to some sub-optimal register allocation. 
For this example, the only difference in the output is that the assignments for
r7 and r8 have been switched, but the result is significant growth in code size
since r8 requires predominantly 32-bit instructions to be used while r7
requires predominantly 16-bit instructions.

cc1 -fpreprocessed binding2.i -quiet -dumpbase binding2.i -mthumb
-mcpu=cortex-a8 -march=armv7-a -auxbase-strip binding.o -Os -w -version
-fno-short-enums -fgnu89-inline -o binding2.s

In gcc-8 the output was
DefineConnectorBinding:
        @ args = 0, pretend = 0, frame = 0
        @ frame_needed = 0, uses_anonymous_args = 0
        push    {r0, r1, r2, r3, r4, r5, r6, r7, r8, lr}
        mov     r4, r1
        mov     r8, r0
        mov     r1, r2
        mov     r0, r4
        mov     r5, r2
        mov     r7, r3
        bl      LookupBinding
        mov     r6, r0
        cbz     r0, .L2
        ldr     r7, .L5
        mov     r1, r4
        ldr     r0, [r7]         // 16-bit instruction
        bl      GetAtomString
        mov     r1, r5
        mov     r4, r0
        ldr     r0, [r7]         // 16-bit instruction
        bl      GetAtomString
        ldrh    r1, [r6, #8]
        mov     r5, r0
        ldr     r0, [r7]         // 16-bit instruction
        bl      GetAtomString
        ldrh    r3, [r6, #10]
        ldr     r2, .L5+4
        movs    r1, #103
        str     r5, [sp]
        strd    r0, r3, [sp, #4]
        mov     r3, r4
        mov     r0, r8
        bl      SemanticError
        add     sp, sp, #16
        @ sp needed
        pop     {r4, r5, r6, r7, r8, pc}
.L2:
        mov     r3, r7
        mov     r2, r5
        mov     r1, r4
        mov     r0, r8
        bl      NewConnectorBindingTree
        add     sp, sp, #16
        @ sp needed
        pop     {r4, r5, r6, r7, r8, lr}
        b       AddBinding

In gcc-9 we get

DefineConnectorBinding:
        @ args = 0, pretend = 0, frame = 0
        @ frame_needed = 0, uses_anonymous_args = 0
        push    {r0, r1, r2, r3, r4, r5, r6, r7, r8, lr}
        mov     r4, r1
        mov     r7, r0
        mov     r1, r2
        mov     r0, r4
        mov     r5, r2
        mov     r8, r3
        bl      LookupBinding
        mov     r6, r0
        cbz     r0, .L2
        ldr     r8, .L5+4
        mov     r1, r4
        ldr     r0, [r8]         // 32-bit instruction
        bl      GetAtomString
        mov     r1, r5
        mov     r4, r0
        ldr     r0, [r8]         // 32-bit instruction
        bl      GetAtomString
        ldrh    r1, [r6, #8]
        mov     r5, r0
        ldr     r0, [r8]         // 32-bit instruction
        bl      GetAtomString
        ldrh    r3, [r6, #10]
        ldr     r2, .L5
        movs    r1, #103
        str     r5, [sp]
        strd    r0, r3, [sp, #4]
        mov     r3, r4
        mov     r0, r7
        bl      SemanticError
        add     sp, sp, #16
        @ sp needed
        pop     {r4, r5, r6, r7, r8, pc}
.L2:
        mov     r3, r8
        mov     r2, r5
        mov     r1, r4
        mov     r0, r7
        bl      NewConnectorBindingTree
        add     sp, sp, #16
        @ sp needed
        pop     {r4, r5, r6, r7, r8, lr}
        b       AddBinding

R8 is used more often than R7, so it seems odd that it is preferred over the
latter.