Hi. Ultra newbie here. This program does work, despite exceeding the available RAM (2K) on my Nano:
int b[10000];
void setup() {
Serial.begin( 115200 );
while( !Serial );
for(int i = 0; i < 10000; ++i)
b[i] = i;
for(int i = 0; i < 10000; ++i)
Serial.println(i);
}
void loop() {
}
The compiler states:
How is it possible? I tried to search this forum and the internet, without any success.
That is not a particularly severe test. Try:
Serial.println( b[ i ] ) ;
A compiler optimisation has exclude the worst of it.
The optimizer notices you don’t use the b array in any meaningful way and thus gets rid of that code altogether
OK, thank you, now begins to became clearer. So the rule is: you can declare whatever you want; you can even write to it; but the compiler warns/stops you only when you try to read from it. Am I correct?
Could you please suggest a link to documentation that explains deeply how the optimizer works? Thank you
If you are indeed an "Ultra newbie", a deeply explanation of the optimizer is going to be a giant waste of you time.
Just sayin'.
a7
I'm an ultra newbie, but in Arduino stuff. Anyway thank you for wasting your time answering with no additional useful info.
It's all here and should be more or less self explanatory: Optimize Options (Using the GNU Compiler Collection (GCC))
Arduino for boards like the Uno etc. is currently at C++11
Thank you!
And compiler uses -O3
So now you just have to look at what all those flags do (if that explains @alto777’s remark)
-fauto-inc-dec
-fbranch-count-reg
-fcombine-stack-adjustments
-fcompare-elim
-fcprop-registers
-fdce
-fdefer-pop
-fdelayed-branch
-fdse
-fforward-propagate
-fguess-branch-probability
-fif-conversion
-fif-conversion2
-finline-functions-called-once
-fipa-modref
-fipa-profile
-fipa-pure-const
-fipa-reference
-fipa-reference-addressable
-fmerge-constants
-fmove-loop-invariants
-fmove-loop-stores
-fomit-frame-pointer
-freorder-blocks
-fshrink-wrap
-fshrink-wrap-separate
-fsplit-wide-types
-fssa-backprop
-fssa-phiopt
-ftree-bit-ccp
-ftree-ccp
-ftree-ch
-ftree-coalesce-vars
-ftree-copy-prop
-ftree-dce
-ftree-dominator-opts
-ftree-dse
-ftree-forwprop
-ftree-fre
-ftree-phiprop
-ftree-pta
-ftree-scev-cprop
-ftree-sink
-ftree-slsr
-ftree-sra
-ftree-ter
-funit-at-a-time
-falign-functions -falign-jumps
-falign-labels -falign-loops
-fcaller-saves
-fcode-hoisting
-fcrossjumping
-fcse-follow-jumps -fcse-skip-blocks
-fdelete-null-pointer-checks
-fdevirtualize -fdevirtualize-speculatively
-fexpensive-optimizations
-ffinite-loops
-fgcse -fgcse-lm
-fhoist-adjacent-loads
-finline-functions
-finline-small-functions
-findirect-inlining
-fipa-bit-cp -fipa-cp -fipa-icf
-fipa-ra -fipa-sra -fipa-vrp
-fisolate-erroneous-paths-dereference
-flra-remat
-foptimize-sibling-calls
-foptimize-strlen
-fpartial-inlining
-fpeephole2
-freorder-blocks-algorithm=stc
-freorder-blocks-and-partition -freorder-functions
-frerun-cse-after-loop
-fschedule-insns -fschedule-insns2
-fsched-interblock -fsched-spec
-fstore-merging
-fstrict-aliasing
-fthread-jumps
-ftree-builtin-call-dce
-ftree-loop-vectorize
-ftree-pre
-ftree-slp-vectorize
-ftree-switch-conversion -ftree-tail-merge
-ftree-vrp
-fvect-cost-model=very-cheap
-fgcse-after-reload
-fipa-cp-clone
-floop-interchange
-floop-unroll-and-jam
-fpeel-loops
-fpredictive-commoning
-fsplit-loops
-fsplit-paths
-ftree-loop-distribution
-ftree-partial-pre
-funswitch-loops
-fvect-cost-model=dynamic
-fversion-loops-for-strides
Don't be rude. @alto777's comment was entirely legitimate.
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