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2013-11-13T12:35:39.777
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I can only make a guess here: It seems, the clang optimizer prefers to use "immediate offsets" for accessing the input and output arrays. e.g. <pre><code>str.w r3, [r1, lr, lsl #3] </code></pre> If the offset exceeds 1024, the clang optimizer chooses double register load and stores with a register offset: Here is a disassembly for when the range of accessed integers exceeds 1024 (1025 in this case): <pre><code>0x79188: push {r4, r5, r7, lr} 0x7918a: add r7, sp, #8 0x7918c: str r8, [sp, #-4]! 0x79190: movw r3, #1025 0x79194: ldrd r8, r9, [r0] 0x79198: add.w r5, r9, r2 0x7919c: adds r0, #8 0x7919e: subs r3, #1 0x791a0: add.w r4, r8, r2 0x791a4: strd r4, r5, [r1] 0x791a8: add.w r1, r1, #8 0x791ac: bne 0x79194 ; Add_8K_3 + 12 at AppDelegate.m:24 0x791ae: movs r0, #0 0x791b0: ldr r8, [sp], #4 0x791b4: pop {r4, r5, r7, pc} </code></pre> When the range of accessed integers exceeds 1800 (<code>for (int i = 0; i < 1800; ++i)</code>) clang will again choose to use "immediate offsets" with single register load and stores: <pre><code>0x94180: push {r7, lr} 0x94182: mov.w lr, #0 0x94186: mov r7, sp 0x94188: ldr.w r12, [r0, lr, lsl #3] 0x9418c: add.w r3, r0, lr, lsl #3 0x94190: ldr.w r9, [r3, #4] 0x94194: add.w r3, r12, r2 0x94198: str.w r3, [r1, lr, lsl #3] 0x9419c: add.w r12, r1, lr, lsl #3 0x941a0: add.w lr, lr, #1 0x941a4: add.w r3, r9, r2 0x941a8: str.w r3, [r12, #4] 0x941ac: cmp.w lr, #1800 0x941b0: bne 0x94188 ; Add_8K_3 + 8 at AppDelegate.m:24 0x941b2: movs r0, #0 0x941b4: pop {r7, pc} </code></pre> Not sure, why - maybe heuristics which suggest the best performance.
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1. POHow to do optimization with C code in Xcode
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