In the integer calling convention, reals are passed without sign extension #82
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…nsion The previous wording indicated 32-bit floats should be sign-extended when passed in integer registers (e.g. when using the lp64 ABI): 1) When passed in registers, scalars narrower than XLEN bits are widened according to the sign of their type up to 32 bits, then sign-extended to XLEN bits. 2) Floating-point reals are passed the same way as integers of the same size 3) For aggregates, bits unused due to padding, and bits past the end of an aggregate whose size in bits is not divisible by XLEN, are undefined. This isn't obeyed in GCC (see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87690), and it seems likely that the original wording simply intended to describe the behaviour regarding selecting registers or passing on the stack, rather than implying sign-extension. Reword to say that reals are passed in the same way as aggregates of the same size, which means when the float is narrower than XLEN the extra bits are undefined.
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My understanding is that this new text matches the behavior of GCC, which is the solution we agreed on in that GCC bug. @jim-wilson does this make sense? I haven't looked deeply enough in to this to be sure. |
jim-wilson
approved these changes
Oct 31, 2018
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The previous wording indicated 32-bit floats should be sign-extended
when passed in wider integer registers (e.g. when using the lp64 ABI):
according to the sign of their type up to 32 bits, then sign-extended to
XLEN bits.
size
an aggregate whose size in bits is not divisible by XLEN, are undefined.
This isn't obeyed in GCC (see
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87690), and it seems likely
that the original wording simply intended to describe the behaviour
regarding selecting registers or passing on the stack, rather than
implying sign-extension.
Reword to say that reals are passed in the same way as aggregates of
the same size, which means when the float is narrower than XLEN the
extra bits are undefined.