------- Comment #4 from ramana at gcc dot gnu dot org 2009-07-02 09:39 -------
(In reply to comment #3)
> Is there a C test case? Can you add objdump of the gcc-generated asm and the
> fixed asm to show the impact on code size? (/me is surprised that 3*"add
> r0,sp,4" is smaller than 1**"add r0,sp,4"+3*"mov r0,r4"... Thumb is amazing
> :-)
The length of add r0,sp,4 and mov r0,r4 is the same for Thumb1 (16 bits).
I suppose the ideal code generated would be something like this modulo errors
with stack alignments in the prologue and the epilogue.
We also don't need r4 in that case :) . So we can save a load, a store as well
as 1 instruction over all. Smaller and faster by 1 instruction and reduced
register usage.
push {lr}
sub sp, sp, #12 (8 byte stack alignment )
add r0, sp, 4 // add r0, sp, 4
bl _ZN1XC1Ev
add r0, sp, #4 // add r0, sp, 4
bl _Z3barP1X
add r0, sp, #4 // add r0, sp, 4
bl _ZN1XD1Ev
add sp, sp, #12 (8 byte stack alignment )
@ sp needed for prologue
pop {pc}
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40615
