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Re: lib/51530: ldexp(2.0,INT_MAX) still zero on mips64el
The following reply was made to PR lib/51530; it has been noted by GNATS.
Subject: Re: lib/51530: ldexp(2.0,INT_MAX) still zero on mips64el
Date: Thu, 6 Oct 2016 01:27:46 +0000
MIPS has its own implementation of ldexp: lib/libc/arch/mips/gen/ldexp.S
71 addu t1, t1, a2 # scale exponent
72 sll v0, a2, 20 # position N for addition
73 bge t1, DEXP_INF, 8f # overflow?
74 addu v0, v0, v1 # multiply by (2**N)
75 ble t1, zero, 4f # underflow?
The exponent will never underflow, because addu is 'add unsigned'.
But even if changed to add, we run into a surprise:
According to MIPS Architecture For Programmers Volume II-A: The MIPS64
Instruction Set  page 51,
If the addition results in 32-bit 2's complement arithmetic overflow, the
destination register is not modified and an Integer Overflow exception occurs
This behaviour is different from the expected integer overflow
 - http://wiki.prplfoundation.org/w/images/6/6f/MD00087-2B-MIPS64BIS-AFP-05.04.pdf
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