[llvm-dev] struct bitfield regression between 3.6 and 3.9 (using -O0)

[Friedman, Eli via llvm-dev]

On 12/22/2016 5:45 PM, Phil Tomson wrote:
> Given that this is compiled with -O0, would there a way to skip the 
> Optimization of the Type-legalized selection DAG? It's fine until it 
> optimizes the Type-legalized selection DAG into the Optimized 
> Type-legalized selection DAG.

Umm, I wouldn't really suggest shoving the problem under the rug... I 
mean, turning off the optimization might make this particular testcase 
work the way you want it to, but the problem will still be lurking, 
waiting to be triggered by a different configuration.

There are patches floating around to turn off DAGCombine, and various 
parts of it, at -O0; you should be able to find past email threads on 
llvmdev discussing it.  IIRC it causes problems for various targets 
because it exercises different codepaths.

-Eli

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