[llvm-commits] Patch to safely restore optimization to testcase from PR3225 on x86

[Eli Friedman]

A little while ago, I committed a patch for PR3225 which disables a
dangerous optimization for expanding double-width shifts (e.g. 64-bit
shifts on x86).  This significantly pessimized the code generated on
x86 for the cases which the optimization covered.

The attached performs a similar optimization in x86 platform-specific
code, which significantly improves code quality for the cases it
covers; this version is safe because SHLD/SHRD do the right thing for
shifts with a zero amount.

Any comments?

-Eli
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