On July 17, 2021 8:54:38 PM GMT+02:00, Stefan Kanthak <stefan.kant...@nexgo.de>
wrote:
>Hi,
>
>GCC 10.2.0 (and GCC 8.3; other versions and targets except i386 and
>amd64 not tested) generate rather bad code for the following ternary
>expression:
>
>--- repro.c ---
>#define NULL (char *) 0
>
>char *dummy(char *string, long count) {
> return count == 0 ? NULL : string + 1;
>}
>--- EOF ---
>
>$ gcc -m64 -o- -O3 -S repro.c
>
>dummy:
> addq $1, %rdi
> movl $0, %eax
> testq %rsi, %rsi
> cmovne %rdi, %rax
> ret
>
>JFTR: why does GCC NOT generate the shorter "XOR %eax, %eax" here?
>
>$ gcc -m64 -O3 -c dummy.c
>$ objdump -D dummy.o
>
>0000000000000000 <dummy>:
> 0: 48 83 c7 01 add $0x1,%rdi
> 4: b8 00 00 00 00 mov $0x0,%eax
> 9: 48 85 f6 test %rsi,%rsi
> c: 48 0f 45 c7 cmovne %rdi,%rax
> 10: c3 retq
>
>
>i386 and AMD64 use the ILP32 and LP64 data model where a "long" and
>a "pointer" have the same size, and 0L and the null pointer have the
>same binary representation, so the contents of RSI should be used to
>load RAX with 0 conditionally:
>
>dummy:
> leaq 1(%rdi), %rax
> testq %rsi, %rsi
> cmoveq %rdi, %rax
> ret
>
>$ gcc -m32 -o- -O3 -S dummy.c
>
>_dummy:
> movl 8(%esp), %edx
> movl 4(%esp), %eax
> addl $1, %eax
> testl %edx, %edx
> movl $0, %edx
> cmove %edx, %eax # OUCH: if this executes, EDX was 0 before,
> ret # so the MOV is really a NOP!
>
>
>$ gcc -m32 -O3 -c dummy.c
>$ objdump -D dummy.o
>
>00000000 <_dummy>:
> 0: 8b 54 24 08 mov 0x8(%esp),%edx
> 4: 8b 44 24 04 mov 0x4(%esp),%eax
> 8: 83 c0 01 add $0x1,%eax
> b: 85 d2 test %edx,%edx
> d: ba 00 00 00 00 mov $0x0,%edx
> 12: 0f 44 c2 cmove %edx,%eax
> 15: c3 ret
>
>Here's what GCC should but generate:
>
>00000000 <_dummy>:
> 0: 8b 44 24 04 mov 0x4(%esp),%eax
> 4: 8b 4c 24 08 mov 0x8(%esp),%ecx
> 8: 40 inc %eax
> 9: f7 d9 neg %ecx
> b: 19 c9 sbb %ecx,%ecx
> d: 21 c8 and %ecx,%eax
> f: c3 ret
>
>
>For (pre)historic processors which don't support CMOVcc the
>following code is generated:
>
>$ gcc -m32 -mtune=i386 -o- -S dummy.c
>
>_dummy:
> movl 8(%esp), %eax
> testl %eax, %eax
> je L3
> movl 4(%esp), %eax
> incl %eax
> ret
> .p2align 2
>L3: # OUCH: EAX is already 0 here!
> xorl %eax, %eax
> ret
>
>00000000 <dummy>:
> 0: 8b 44 24 08 mov 0x8(%esp),%eax
> 4: 85 c0 test %eax,%eax
> 6: 74 08 je 10 <dummy+0x10>
> 8: 8b 44 24 04 mov 0x4(%esp),%eax
> c: 40 inc %eax
> d: c3 ret
> e: 66 90 xchg %ax,%ax
> 10: 31 c0 xor %eax,%eax
> 12: c3 ret
>
>
>not amused
Patches welcome. You might want to file a bugzilla report which has a higher
chance of being found after a while.
Richard.
>Stefan Kanthak
