Bug 9991 – Optimizer Doesn't Enregister Static Array Elements With Compile-Time Constant Index

Can't static array elements where the index is known at compile time be treated just like regular stack variables? GDC seems to treat them as such. D Source code below. I'd expect these two implementations to compile to the exact same ASM. In GDC, the main loops do, though the pre-loop setup stuff does compile differently for reasons I don't understand. int sum1(const int[] arr) { auto end = arr.ptr + arr.length - 3; int[4] sums; for(auto cur = arr.ptr; cur < end; cur += 4) { sums[0] += cur[0]; sums[1] += cur[1]; sums[2] += cur[2]; sums[3] += cur[3]; } return sums[0] + sums[1] + sums[2] + sums[3]; } int sum2(const int[] arr) { auto end = arr.ptr + arr.length - 3; int sum0 = 0, sum1 = 0, sum2 = 0, sum3 = 0; for(auto cur = arr.ptr; cur < end; cur += 4) { sum0 += cur[0]; sum1 += cur[1]; sum2 += cur[2]; sum3 += cur[3]; } return sum0 + sum1 + sum2 + sum3; } ASM: _D5test34sum1FxAiZi: push RBP mov RBP,RSP sub RSP,020h push RBX lea RAX,-020h[RBP] xor RCX,RCX mov [RAX],RCX mov 8[RAX],RCX mov R8,RSI mov RDX,RSI mov R9,RDI lea R9,0FFFFFFF4h[R9*4][RDX] cmp RDX,R9 jae L51 L2D: mov EBX,[R8] add -020h[RBP],EBX mov ESI,4[R8] add -01Ch[RBP],ESI mov EAX,8[R8] add -018h[RBP],EAX mov ECX,0Ch[R8] add -014h[RBP],ECX add R8,010h cmp R8,R9 jb L2D L51: mov EAX,-020h[RBP] add EAX,-01Ch[RBP] add EAX,-018h[RBP] add EAX,-014h[RBP] pop RBX mov RSP,RBP pop RBP ret nop _D5test34sum2FxAiZi: push RBP mov RBP,RSP sub RSP,010h push RBX push R12 push R13 xor R9D,R9D xor R11D,R11D xor EBX,EBX xor R12D,R12D mov R8,RSI mov RAX,RSI mov R13,RDI lea R13,0FFFFFFF4h[R13*4][RAX] cmp RAX,R13 jae L46 L2E: add R9D,[R8] add R11D,4[R8] add EBX,8[R8] add R12D,0Ch[R8] add R8,010h cmp R8,R13 jb L2E L46: lea EAX,[R11][R9] add EAX,EBX add EAX,R12D pop R13 pop R12 pop RBX mov RSP,RBP pop RBP ret

The optimizer currently does not enregister variables that don't fit into registers. It could be enhanced to do it.

(In reply to comment #1) > The optimizer currently does not enregister variables that don't fit into > registers. It could be enhanced to do it. ints definitely fit into registers.

(In reply to comment #2) > ints definitely fit into registers. sums is one variable, not 4.

(In reply to comment #0) > In GDC, the main loops do, though the pre-loop setup stuff > does compile differently for reasons I don't understand. ldc2 compiles them to the same asm (-release -profile-verifier-noassert -O5 -output-s): __D4test4sum1FxAiZi: pushl %ebx pushl %edi pushl %esi movl 16(%esp), %eax movl 20(%esp), %esi leal -12(%esi,%eax,4), %ebx xorl %ecx, %ecx xorl %edx, %edx xorl %edi, %edi xorl %eax, %eax jmp LBB0_2 .align 16, 0x90 LBB0_1: addl 12(%esi), %eax addl 8(%esi), %edi addl 4(%esi), %edx addl (%esi), %ecx addl $16, %esi LBB0_2: cmpl %ebx, %esi jb LBB0_1 addl %edi, %eax addl %edx, %eax addl %ecx, %eax popl %esi popl %edi popl %ebx ret $8 __D4test4sum2FxAiZi: pushl %ebx pushl %edi pushl %esi movl 16(%esp), %eax movl 20(%esp), %esi leal -12(%esi,%eax,4), %ebx xorl %ecx, %ecx xorl %edx, %edx xorl %edi, %edi xorl %eax, %eax jmp LBB1_2 .align 16, 0x90 LBB1_1: addl 12(%esi), %ecx addl 8(%esi), %edx addl 4(%esi), %edi addl (%esi), %eax addl $16, %esi LBB1_2: cmpl %ebx, %esi jb LBB1_1 addl %edi, %eax addl %edx, %eax addl %ecx, %eax popl %esi popl %edi popl %ebx ret $8

FWIW, after https://github.com/dlang/dmd/pull/6176 (2.073.0) the generated assembly code changed, however a simple benchmark shows that performance did not noticeably change.

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