Reduce clang-generated code size for ARM - c

Question

I compare code generated by clang and generated by gcc for arm.
Unfortunately, gcc's code more often has less instructions.
I am just curious - is it possible to reduce code, generated by clang?
Maybe I should use some options to do so...
Please, consider very simple example:
> cat test.c
int to_upper(int c)
{
if(c < 'a' || c > 'z') return c;
else return c - ('a' - 'A');
}
> clang -target arm-none-eabi -Oz -c -mthumb -mcpu=cortex-m0 -msoft-float ./test.c -o ./clang_test.o
> /usr/bin/arm-none-eabi-gcc -Os -c -mthumb -mcpu=cortex-m0 -msoft-float ./test.c -o ./gcc_test.o
> /usr/bin/arm-none-eabi-objdump -d ./clang_test.o
./clang_test.o: file format elf32-littlearm
Disassembly of section .text:
00000000 <to_upper>:
0: 4602 mov r2, r0
2: 3a61 subs r2, #97 ; 0x61
4: 4601 mov r1, r0
6: 3920 subs r1, #32
8: 2a19 cmp r2, #25
a: d800 bhi.n e <to_upper+0xe>
c: 4608 mov r0, r1
e: 4770 bx lr
> /usr/bin/arm-none-eabi-objdump -d ./gcc_test.o
./gcc_test.o: file format elf32-littlearm
Disassembly of section .text:
00000000 <to_upper>:
0: 1c03 adds r3, r0, #0
2: 3b61 subs r3, #97 ; 0x61
4: 2b19 cmp r3, #25
6: d800 bhi.n a <to_upper+0xa>
8: 3820 subs r0, #32
a: 4770 bx lr
Why so much difference in such simple code?
Can clang generate less code in this case? At least as gcc?
Note: if we change cpu to -mcpu=cortex-a5 (other options remains the same), then clang ang gcc produce
absolutely identical code:
00000000 <to_upper>:
0: f1a0 0361 sub.w r3, r0, #97 ; 0x61
4: 2b19 cmp r3, #25
6: bf98 it ls
8: 3820 subls r0, #32
a: 4770 bx lr
OS: Ubuntu 14.04.3
clang version 3.7.1 (tags/RELEASE_371/final)
Target: x86_64-unknown-linux-gnu
Thread model: posix
arm-none-eabi-gcc (4.8.2-14ubuntu1+6) 4.8.2

Answer 1

No, clang cannot generate less code in this case. And also in many others.
Historically, very few code size optimizations have been implemented in LLVM. When optimizing for code size, GCC typically outperforms LLVM significantly.
Here presentation, where done a closer look at the comparing GCC and Clang in terms of code size optimization.
Presentation video