Bug 14436 – Optimizer fails to remove comparison loop when comparing array against null

The comparison in this code int[] arr; void main() { if (arr != null) { arr = [1]; } } gets lowered to !arr.length && !memcmp(arr.ptr, null, arr.length) The optimizer should be able to tell that the memcmp would always return 0 and skip the check.

Commits pushed to master at https://github.com/D-Programming-Language/dmd https://github.com/D-Programming-Language/dmd/commit/7660158f16b9c988d8ff16592ee442051ea95ffa Fix Issue 14436 - Optimizer fails to remove comparison loop when comparing array against null https://github.com/D-Programming-Language/dmd/commit/6f5a3cde5778e263d53d49f4546b668652ca89ae Merge pull request #4574 from yebblies/issue14436 Issue 14436 - Optimizer fails to remove comparison loop when comparing array against null

Comparing with an empty array or string literal still generates suboptimal code: bool isEmpty(string s) { return s == ""; } # dmd -c -O -inline -release xx.d 0000000000000000 <_D2xx7isEmptyFAyaZb>: 0: 55 push %rbp 1: 48 8b ec mov %rsp,%rbp 4: 48 83 ec 10 sub $0x10,%rsp 8: 48 89 7d f0 mov %rdi,-0x10(%rbp) c: 48 89 75 f8 mov %rsi,-0x8(%rbp) 10: 48 3b 3d 00 00 00 00 cmp 0x0(%rip),%rdi # 17 <_D2xx7isEmptyFAyaZb+0x17> 17: 75 15 jne 2e <_D2xx7isEmptyFAyaZb+0x2e> 19: 48 8b 75 f8 mov -0x8(%rbp),%rsi 1d: 48 8b 3d 00 00 00 00 mov 0x0(%rip),%rdi # 24 <_D2xx7isEmptyFAyaZb+0x24> 24: 48 8b 4d f0 mov -0x10(%rbp),%rcx 28: 33 c0 xor %eax,%eax 2a: f3 a6 repz cmpsb %es:(%rdi),%ds:(%rsi) 2c: 74 04 je 32 <_D2xx7isEmptyFAyaZb+0x32> 2e: 31 c0 xor %eax,%eax 30: eb 05 jmp 37 <_D2xx7isEmptyFAyaZb+0x37> 32: b8 01 00 00 00 mov $0x1,%eax 37: 48 8b e5 mov %rbp,%rsp 3a: 5d pop %rbp 3b: c3 retq Reopening, as it is a closely related enhancement (though strictly speaking not identical).

By empty array do you mean empty array literal? eg assert(x != []);

(In reply to yebblies from comment #4) > By empty array do you mean empty array literal? > > eg assert(x != []); Yes, it should read "empty array [literal] or string literal". In fact, the code generated for `int[]` instead of `string` looks even worse; there is a "call" instruction in it: 0000000000000000 <_D2xx7isEmptyFAiZb>: 0: 55 push %rbp 1: 48 8b ec mov %rsp,%rbp 4: 48 83 ec 30 sub $0x30,%rsp 8: 48 89 7d f0 mov %rdi,-0x10(%rbp) c: 48 89 75 f8 mov %rsi,-0x8(%rbp) 10: 48 31 f6 xor %rsi,%rsi 13: 48 bf 00 00 00 00 00 movabs $0x0,%rdi 1a: 00 00 00 1d: e8 00 00 00 00 callq 22 <_D2xx7isEmptyFAiZb+0x22> 22: 48 89 c1 mov %rax,%rcx 25: 48 31 c0 xor %rax,%rax 28: 48 89 45 e0 mov %rax,-0x20(%rbp) 2c: 48 89 4d e8 mov %rcx,-0x18(%rbp) 30: 48 89 45 d0 mov %rax,-0x30(%rbp) 34: 48 89 4d d8 mov %rcx,-0x28(%rbp) 38: 48 8b 55 f0 mov -0x10(%rbp),%rdx 3c: 48 3b 55 e0 cmp -0x20(%rbp),%rdx 40: 75 1a jne 5c <_D2xx7isEmptyFAiZb+0x5c> 42: 48 85 d2 test %rdx,%rdx 45: 74 19 je 60 <_D2xx7isEmptyFAiZb+0x60> 47: 48 8b 75 f8 mov -0x8(%rbp),%rsi 4b: 48 8b 7d d8 mov -0x28(%rbp),%rdi 4f: 48 89 d1 mov %rdx,%rcx 52: 48 c1 e1 02 shl $0x2,%rcx 56: 33 c0 xor %eax,%eax 58: f3 a6 repz cmpsb %es:(%rdi),%ds:(%rsi) 5a: 74 04 je 60 <_D2xx7isEmptyFAiZb+0x60> 5c: 31 c0 xor %eax,%eax 5e: eb 05 jmp 65 <_D2xx7isEmptyFAiZb+0x65> 60: b8 01 00 00 00 mov $0x1,%eax 65: 48 8b e5 mov %rbp,%rsp 68: 5d pop %rbp 69: c3 retq

Ok, I'll have a look. This can probably be special-cased, although there is a long-standing problem that the optimizer doesn't look at constant data correctly.

Commits pushed to stable at https://github.com/D-Programming-Language/dmd https://github.com/D-Programming-Language/dmd/commit/7660158f16b9c988d8ff16592ee442051ea95ffa Fix Issue 14436 - Optimizer fails to remove comparison loop when comparing array against null https://github.com/D-Programming-Language/dmd/commit/6f5a3cde5778e263d53d49f4546b668652ca89ae Merge pull request #4574 from yebblies/issue14436

Erroneously closed by the bot, reopening. Latest DMD still produces the assembly in comment #3.

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