https://llvm.org/bugs/show_bug.cgi?id=28658
Bug ID: 28658
Summary: MemorySSA non linearity
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: Scalar Optimizations
Assignee: unassignedb...@nondot.org
Reporter: seb...@gmail.com
CC: david.majne...@gmail.com, dber...@dberlin.org,
george.burgess...@gmail.com, llvm-bugs@lists.llvm.org
Classification: Unclassified
While investigating the cause of a huge number of queries to the MemSSA in the
test-suite, I found that the compilation of
MultiSource/Applications/JM/ldecod/macroblock.c
has an unusually high number of lookups:
436 memoryssa - Number of Memory SSA version cache hits
30401 memoryssa - Number of MemorySSA version cache inserts
128166 memoryssa - Number of Memory SSA version cache
lookups
>From that file I extracted something that exposes the non linearity in the
number of lookups.
$ cat fun.c
#define loop do { \
for (j=0;j<m;j++) \
A[i+j] += B[i+j] * C[i][H[i+j]]; \
} while (0)
void fun(int *A, int **C, int *H, int n, int m) {
int i, j;
for (i=0;i<n;i++) {
int *B = A[i] ? C[i] : C[i+1];
loop; loop; loop; loop; loop; loop; loop; loop; loop; loop;
loop; loop; loop; loop; loop; loop; loop; loop; loop; loop;
}
}
$ clang -O3 -mllvm -stats fun.c
10284 memoryssa - Number of MemorySSA version cache inserts
20569 memoryssa - Number of Memory SSA version cache lookups
Number of Memory SSA version cache lookups:
80 loops - 20569
40 loops - 5489
20 loops - 1549
10 loops - 479
1 loop - 29
Adding another statement in the loop:
#define loop do { \
for (j=0;j<m;j++) \
A[i+j] += B[i+j] * C[i][H[i+j]]; \
A[i-j] += B[i-j] * C[i+j][H[i-j]]; \
} while (0)
Number of Memory SSA version cache lookups:
80 loops - 160824
40 loops - 42024
20 loops - 11424
10 loops - 3324
1 loop - 138
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