Regression in StructurizeCFG, r321751: complex loop structure turned into infinite loop

[nhaehnle]

Bugzilla Link	36015
Resolution	FIXED
Resolved on	Jan 30, 2018 03:36
Version	trunk
OS	Linux
Blocks	#35152
Attachments	Reduced test case, Output of opt -structurizecfg
CC	@zmodem,@arsenm

Extended Description

I'm attaching a reduced test case, plus the output of opt -structurizecfg.

Change r321751 ("StructurizeCFG: Fix broken backedge detection") breaks this test case, in the following way:

The original code has edges from both %if and %else into the %exit block. In the resulting code, the edge from %if to %exit disappears: if the exit condition is met (%cond.if = false), the control will follow %if -> %Flow1 -> %Flow2 -> %loop.outer instead of %if -> %Flow1 -> %Flow2 -> %exit.

This bug regresses various OpenGL test suites, for example dEQP-GLES3.functional.shaders.loops.do_while_dynamic_iterations.conditional_continue_vertex.

[nhaehnle]

I have now reverted the breaking change in trunk, r323355, but this change also needs to be backed out of the release branch.

[zmodem]

I have now reverted the breaking change in trunk, r323355, but this change
also needs to be backed out of the release branch.

Merged the revert in r323749.

[nhaehnle]

Thank you!