This reduces the size of iommu_gas.o by 864 bytes.
This program {F46792064}, compiled with -DPLAIN -O2 on a friendly machine, produced measurable performance improvements:
```
x times
+ times.orig
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||A A |
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N Min Max Median Avg Stddev
x 48 80.543295 80.613294 80.585076 80.583921 0.017909773
+ 48 86.499218 86.599074 86.534515 86.537865 0.018445631
Difference at 95.0% confidence
5.95394 +/- 0.00736987
7.3885% +/- 0.00914558%
(Student's t, pooled s = 0.0181797)
```
Other machines produce different results.