Nikita Popov a3f725f6cd Reapply "[BDCE][DemandedBits] Detect dead uses of undead instructions" ... This (mostly) fixes https://bugs.llvm.org/show_bug.cgi?id=39771. BDCE currently detects instructions that don't have any demanded bits and replaces their uses with zero. However, if an instruction has multiple uses, then some of the uses may be dead (have no demanded bits) even though the instruction itself is still live. This patch extends DemandedBits/BDCE to detect such uses and replace them with zero. While this will not immediately render any instructions dead, it may lead to simplifications (in the motivating case, by converting a rotate into a simple shift), break dependencies, etc. The implementation tries to strike a balance between analysis power and complexity/memory usage. Originally I wanted to track demanded bits on a per-use level, but ultimately we're only really interested in whether a use is entirely dead or not. I'm using an extra set to track which uses are dead. However, as initially all uses are dead, I'm not storing uses those user is also dead. This case is checked separately instead. The previous attempt to land this lead to miscompiles, because cases where uses were initially dead but were later found to be live during further analysis were not always correctly removed from the DeadUses set. This is fixed now and the added test case demanstrates such an instance. Differential Revision: https://reviews.llvm.org/D55563 llvm-svn: 350188		2019-01-01 10:05:26 +00:00
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llvm	Reapply "[BDCE][DemandedBits] Detect dead uses of undead instructions"	2019-01-01 10:05:26 +00:00
llvm-c	[IR] Add a dedicated FNeg IR Instruction	2018-11-13 18:15:47 +00:00