http://llvm.org/bugs/show_bug.cgi?id=10066
Summary: Greedy allocator fails to fold a memory operand when
spilling
Product: libraries
Version: trunk
Platform: PC
OS/Version: All
Status: NEW
Severity: normal
Priority: P
Component: Register Allocator
AssignedTo: [email protected]
ReportedBy: [email protected]
CC: [email protected]
Created an attachment (id=6676)
--> (http://llvm.org/bugs/attachment.cgi?id=6676)
LLVM IR source
Hi.
The code produced for the test in attachment is worse with the greedy allocator
than it was with the linear scan allocator (note that the test is compiled with
-disable-fp-elim option).
The test is a pair of nested loops. The outer loop counter (in vreg17) is
spilled on x86 but despite being used only once (for decrementing) stack slot
is not folded into the instruction. This happens because vreg17 is first split
into two registers (vreg54 and vreg53). Physical register is allocated for the
first one and the second one is spilled, so at that point it is not possible to
fold the memory operand unless it is somehow known that the two registers
originated from a common one.
Seems that greedy allocator is too greedy at splitting the intervals :) This
might not be a problem for a RISC architecture but it is not very good for
CISC.
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