The same switch statement handles code generation for signed division of
words, double words and quad words. Rather than using multiple switch
statements to select the appropriate instructions, determine all of the
correctly sized operands up front, then use them as needed.

Updates #59089

Change-Id: I2b7567c8e0ecb9904c37607332538c95b0521dca
Reviewed-on: https://go-review.googlesource.com/c/go/+/482657
Run-TryBot: Joel Sing <joel@sing.id.au>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

In order to avoid a CPU exception resulting from signed overflow, the signed
division code tests if the divisor is -1 and if it is, runs fix up code to
manually compute the quotient and remainder (thus avoiding IDIV and potential
signed overflow).

However, the way that this is currently structured means that the normal code
path for the case where the divisor is not -1 results in five instructions
and two branches (CMP, JEQ, followed by sign extension, IDIV and another JMP
to skip over the fix up code).

Rework the fix up code such that the final JMP is incurred by the less likely
divisor is -1 code path, rather than more likely code path (which is already
more expensive due to IDIV). This result in a four instruction sequence
(CMP, JNE, sign extension, IDIV), with only a single branch.

Updates #59089

Change-Id: Ie8d065750a178518d7397e194920b201afeb0530
Reviewed-on: https://go-review.googlesource.com/c/go/+/482658
Run-TryBot: Joel Sing <joel@sing.id.au>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

This adds benchmarks for division and modulus of 64 bit signed and unsigned
integers.

Updates #59089

Change-Id: Ie757c6d74a1f355873e79619eae26ece21a8f23e
Reviewed-on: https://go-review.googlesource.com/c/go/+/482656
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Joel Sing <joel@sing.id.au>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>