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Does gcc hoist memory-reference based loop invariant?

0 votes
95 views

For the below simple case, I think poly1[i] should be hoisted to outermost loop to avoid loading from innermost loop at each iteration.
But for arm-none-eabi target like cortex-m4, gcc fails to do so. I this a normal case or a missing optimization? Please advise.

void
PolyMul (float *poly1, unsigned int n1,
 float *poly2, unsigned int n2,
 float *polymul, unsigned int *nmul)
{
 unsigned int i, j;

 for (i = 0; i for (j = 0; j polymul[i+j] += poly1[i] * poly2[j];
}
posted Jun 13, 2013 by anonymous

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1 Answer

0 votes

Unless you use 'restrict' qualifier to the pointer type, I don't think compiler can do such optimization due to aliasing issue.

answer Jun 13, 2013 by anonymous
Also I don't think -fstrict-aliasing could help for your case. Becase poly1, poly2, and polymul are all 'float *' pointer type.
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+2 votes

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0 votes

I've noticed the difference in gcc and llvm behaviour with the following code:

$ cat test.c

int main()
{
 for(int i = 0;; ({break;}))
 printf("Hello, worldn");
}

$ clang test.c -pedantic &; ({break;}))
1 warning generated.
Hello, world

$ gcc test.c -std=gnu11 -pedantic &; ({break;}))
test.c:5:21: warning: ISO C forbids braced-groups within expressions [-Wpedantic]
 for(int i = 0;; ({break;}))

So, llvm thinks that this is GNU extension (seems like it really is), but compiles it, and gcc does not, even if the standard is specified as gnu11. Is it a bug?

+2 votes

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