Treatment of INTEGER arguments smaller than eightbytes

[fweimer]

It is currently not clear if the excess bits are zero, determined by sign extension, or undefined.

For example, if a function returns type int, the upper 32 bits of %rax could be:

• always zero
• all ones if the return value (as an int) is negative, zero otherwise
• unspecified

[pcordes]

According to what I can dig up:

• Return values have garbage outside the width of the return type.
• Function args are different: narrow types are sign or zero extended to 32 bits, depending on their type. clang depends on this for incoming args, but gcc doesn't make that assumption. But both compilers do it when calling, so gcc code can call clang code.

The upper 32 bits of registers are always undefined garbage for types smaller than 64 bits.

[hjl-tools]

The x86-64 psABI issue should be discussed at

https://groups.google.com/forum/#!forum/x86-64-abi