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cmd/compile: ensure constant shift amounts are in range for arm

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Ensure constant shift amounts are in the range [0-31]. When shift amounts
are out of range, bad things happen. Shift amounts out of range occur
when lowering 64-bit shifts (we take an in-range shift s in [0-63] and
calculate s-32 and 32-s, both of which might be out of [0-31]).

The constant shift operations themselves still work, but their shift
amounts get copied unmolested to operations like ORshiftLL which use only
the low 5 bits. That changes an operation like <<100 which unconditionally
produces 0, to <<4, which doesn't.

Fixes #48476

Change-Id: I87363ef2b4ceaf3b2e316426064626efdfbb8ee3
Reviewed-on: https://go-review.googlesource.com/c/go/+/350969
Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
This commit is contained in:
Keith Randall 2021-09-19 21:09:57 -07:00
parent 9ebe7c8ec6
commit eff27e858b
5 changed files with 318 additions and 68 deletions
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2
src/cmd/compile/internal/ssa/gen/genericOps.go
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@ -106,7 +106,7 @@ var genericOps = []opData{
// For shifts, AxB means the shifted value has A bits and the shift amount has B bits.
// Shift amounts are considered unsigned.
// If arg1 is known to be less than the number of bits in arg0,
// If arg1 is known to be nonnegative and less than the number of bits in arg0,
// then auxInt may be set to 1.
// This enables better code generation on some platforms.
{name: "Lsh8x8", argLength: 2, aux: "Bool"}, // arg0 << arg1