41206b61e3
This patch reverts part of r362750 / D62650, which stopped LiveDebugVariables from trimming leading variable location ranges down to only covering those instructions that are in scope. I've observed some circumstances where the number of DBG_VALUEs in a function can be amplified in an un-necessary way, to cover more instructions that are out of scope, leading to very slow compile times. Trimming the range of instructions that the variables cover solves the slow compile times. The specific problem that r362750 tries to fix is addressed by the assignment to RStart that I've added. Any variable location that begins at the first instruction of a block will now be considered to begin at the start of the block. While these sound the same, the have different SlotIndexes, and the register allocator may shoehorn additional instructions in between the two. The test added in the past (wrong_debug_loc_after_regalloc.ll) still works with this modification. live-debug-variables.ll has a range trimmed to not cover the prologue of the function, while dbg-addr-dse.ll has a DBG_VALUE sink past one instruction with no DebugLoc, which is expected behaviour. Differential Revision: https://reviews.llvm.org/D73691
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