[Bug c++/70491] New: slow compilation initializing a VLA

["msebor at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70491

            Bug ID: 70491
           Summary: slow compilation initializing a VLA
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

Compiling a program that initializes a VLA is several times slower that
compiling the same program that initializes an ordinary array.  The following
scripts create a pair of programs, the first initializes 100 arrays, and the
second initializes 100 VLAs.  The output shows the difference between the two
using an unoptimized GCC build on powerpc64le.

The difference between the two with an optimized GCC is less dramatic but the
VLA case is still 3 times slower on x86_64.

While compiling the VLA program is expected to take more cycles it shouldn't
take several times as long as the equivalent program with ordinary arrays.

$ (i=0; init=""; echo > z.cpp; while [ $i -lt 100 ]; do echo "int f$i (int n) {
int a [$i] = { $init }; return a [0]; }" >> z.cpp; init="$init $i,";
i=$((i+1)); done && time /build/gcc-trunk/gcc/xgcc -B /build/gcc-trunk/gcc -S
-Wall -xc++ z.cpp)

real    0m0.545s
user    0m0.543s
sys     0m0.002s
tmp$ (i=0; init=""; echo > z.cpp; while [ $i -lt 100 ]; do echo "int f$i (int
n) { int a [n] = { $init }; return a [0]; }" >> z.cpp; init="$init $i,";
i=$((i+1)); done && time /build/gcc-trunk/gcc/xgcc -B /build/gcc-trunk/gcc -S
-Wall -xc++ z.cpp)

real    0m6.415s
user    0m6.393s
sys     0m0.022s