Question

Visual C++ can emit C4738 warning:

storing 32-bit float result in memory, possible loss of performance

for cases when a 32-bit float is about to be stored in memory instead of being stored in a register.

The description further says using double resolves the issue. I don’t get why the latter is true.

Why is storing float in memory result in performance loss and storing double does not?

Answer 1

The warning combines two issues:

• Floats need to be stored in memory rather than registers, which can reduce performance (because memory is much slower than registers)
• Floats will be rounded (because registers have 64 or 80 bits always, but in memory a float only has 32 bits).

Using double resolves the second issue (at least partially, 64 bits are still less precise than 80 bits), but has no impact on the possible performance loss. Which is why the warning decription mentions TWO remedies:

To resolve this warning and avoid rounding, compile with /fp:fast or
use doubles instead of floats.

To resolve this warning and avoid running out of registers, change the
order of computation and modify your use of inlining