Non-const DoNotOptimize() can't compile when used with some types.
Example of code which can't compile:
char buffer3[3] = "";
benchmark::DoNotOptimize(buffer3);
Error message:
error: impossible constraint in 'asm'
asm volatile("" : "+r"(value) : : "memory");
Introduced in 8545dfb (Fix DoNotOptimize() GCC copy overhead (#1340) (#1410))
The cause is compiler can't work with the +r constraint for types that can't
be placed perfectly in registers. For example, char array[3] can't be perfectly
fit in register on x86_64 so it requires placed in memory but constraint
doesn't allow that.
Solution
- Use +m,r constraint for the small objects so the compiler can decide to use
register or/and memory
- For the big objects +m constraint is used which allows avoiding extra copy
bug(see #1340)
- The same approach is used for the const version of DoNotOptimize()
although the const version works fine with the "r" constraint only.
Using mixed r,m constraint looks more general solution.
See
- Issue #1340 ([BUG] DoNotOptimize() adds overhead with extra copy of argument(gcc))
- Pull request #1410 (Fix DoNotOptimize() GCC copy overhead (#1340) #1410)
- Commit 8545dfb (Fix DoNotOptimize() GCC copy overhead (#1340) (#1410))
* Fix#342: DoNotOptimize causes compile errors on older GCC versions.
DoNotOptimize uses inline assembly contraints to tell
the compiler what the type of the input variable. The 'g'
operand allows the input to be any register, memory, or
immediate integer operand. However this constraint seems
to be too weak on older GCC versions, and certain inputs
will cause compile errors.
This patch changes the constraint to 'X', which is documented
as "any operand whatsoever is allowed". This appears to fix
the issues with older GCC versions.
However Clang doesn't seem to like "X", and will attempt
to put the input into a register even when it can't/shouldn't;
causing a compile error. However using "g" seems to work like
"X" with GCC, so for this reason Clang still uses "g".
* Try alternative formulation to placate GCC
