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Switch-to-lookup tables: set threshold to 3 cases 

There has been an old FIXME to find the right cut-off for when it's worth
analyzing and potentially transforming a switch to a lookup table.

The switches always have two or more cases. I could not measure any speed-up
by transforming a switch with two cases. A switch with three cases gets a nice
speed-up, and I couldn't measure any compile-time regression, so I think this
is the right threshold.

In a Clang self-host, this causes 480 new switches to be transformed,
and reduces the final binary size with 8 KB.

llvm-svn: 199294
This commit is contained in:
Hans Wennborg 2014-01-15 05:00:27 +00:00
parent 5cf521047f
commit 4744ac1733
2 changed files with 39 additions and 5 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

8
llvm/lib/Transforms/Utils/SimplifyCFG.cpp
View File

@ -3684,11 +3684,9 @@ static bool SwitchToLookupTable(SwitchInst *SI,
// GEP needs a runtime relocation in PIC code. We should just build one big
// string and lookup indices into that.
// Ignore the switch if the number of cases is too small.
// This is similar to the check when building jump tables in
// SelectionDAGBuilder::handleJTSwitchCase.
// FIXME: Determine the best cut-off.
if (SI->getNumCases() < 4)
// Ignore switches with less than three cases. Lookup tables will not make them
// faster, so we don't analyze them.
if (SI->getNumCases() < 3)
return false;
// Figure out the corresponding result for each case value and phi node in the

36
llvm/test/Transforms/SimplifyCFG/X86/switch_to_lookup_table.ll
View File

@ -856,3 +856,39 @@ return:
; CHECK-NOT: @switch.table
; CHECK: switch i32
}
; We build lookup tables for switches with three or more cases.
define i32 @threecases(i32 %c) {
entry:
switch i32 %c, label %sw.default [
i32 0, label %return
i32 1, label %sw.bb1
i32 2, label %sw.bb2
]
sw.bb1: br label %return
sw.bb2: br label %return
sw.default: br label %return
return:
%x = phi i32 [ 3, %sw.default ], [ 5, %sw.bb2 ], [ 7, %sw.bb1 ], [ 9, %entry ]
ret i32 %x
; CHECK-LABEL: @threecases(
; CHECK-NOT: switch i32
; CHECK: @switch.table
}
; We don't build tables for switches with two cases.
define i32 @twocases(i32 %c) {
entry:
switch i32 %c, label %sw.default [
i32 0, label %return
i32 1, label %sw.bb1
]
sw.bb1: br label %return
sw.default: br label %return
return:
%x = phi i32 [ 3, %sw.default ], [ 7, %sw.bb1 ], [ 9, %entry ]
ret i32 %x
; CHECK-LABEL: @twocases(
; CHECK: switch i32
; CHECK-NOT: @switch.table
}