[cfe-commits] Fix bug 12212 Missing mul-to-shift conversion for negative non-power-of 2 constants on ARM

[Weiming Zhao]

Hello,

 

LLVM supports converting multiplication to shifting and add/sub operations
if possible when one operand is a integer variable and the other is a
positive constant even if the constant is not power of 2.

For example, "x*5" will be converted to "x+x<<2".

 

However, when the constant  is a negative and not power of 2, it always
emits "mul" even if using shifting+add/sub is possible.

For example, "x*-7" should be converted to "x-x<<8"  (sub r0, r0, r0, lsl #3
)

 

I have reported this into the bugzilla. The URL is 

http://llvm.org/bugs/show_bug.cgi?id=12212

 

The cause of the issue is in ARMISelLowering.cpp, it treats the constant
("MulAmt") as uint64_t and therefore it omits the case when MulAmt could be
negative.

So, I'm providing the fix for this issue. 

I also run the unit tests and the test suite. My fix doesn't change any
result from validtion tests. I believe this is good fix.  :D

 

I'm attaching the fix (0001-Bug12212-*.patch) and an updated test case
(0002-neg-mul-const-unit-test.patch) for such scenario as well as my
validation run results (test_results.tar.bz2).

 

Please review it.

 

Thanks,

Weiming Zhao

 

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