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llvm-mirror/test/Transforms/LowerSwitch/2014-06-10-SwitchContiguousOpt.ll

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LowerSwitch: track bounding range for the condition tree. When LowerSwitch transforms a switch instruction into a tree of ifs it is actually performing a binary search into the various case ranges, to see if the current value falls into one cases range of values. So, if we have a program with something like this: switch (a) { case 0: do0(); break; case 1: do1(); break; case 2: do2(); break; default: break; } the code produced is something like this: if (a < 1) { if (a == 0) { do0(); } } else { if (a < 2) { if (a == 1) { do1(); } } else { if (a == 2) { do2(); } } } This code is inefficient because the check (a == 1) to execute do1() is not needed. The reason is that because we already checked that (a >= 1) initially by checking that also (a < 2) we basically already inferred that (a == 1) without the need of an extra basic block spawned to check if actually (a == 1). The patch addresses this problem by keeping track of already checked bounds in the LowerSwitch algorithm, so that when the time arrives to produce a Leaf Block that checks the equality with the case value / range the algorithm can decide if that block is really needed depending on the already checked bounds . For example, the above with "a = 1" would work like this: the bounds start as LB: NONE , UB: NONE as (a < 1) is emitted the bounds for the else path become LB: 1 UB: NONE. This happens because by failing the test (a < 1) we know that the value "a" cannot be smaller than 1 if we enter the else branch. After the emitting the check (a < 2) the bounds in the if branch become LB: 1 UB: 1. This is because by checking that "a" is smaller than 2 then the upper bound becomes 2 - 1 = 1. When it is time to emit the leaf block for "case 1:" we notice that 1 can be squeezed exactly in between the LB and UB, which means that if we arrived to that block there is no need to emit a block that checks if (a == 1). Patch by: Marcello Maggioni <hayarms@gmail.com> llvm-svn: 211038
2014-06-16 18:55:20 +02:00
; RUN: opt < %s -lowerswitch -S | FileCheck %s
; CHECK-NOT: icmp eq i32 %0, 1
define i32 @foo(i32 %a) #0 {
entry:
%retval = alloca i32, align 4
%a.addr = alloca i32, align 4
store i32 %a, i32* %a.addr, align 4
[opaque pointer type] Add textual IR support for explicit type parameter to load instruction Essentially the same as the GEP change in r230786. A similar migration script can be used to update test cases, though a few more test case improvements/changes were required this time around: (r229269-r229278) import fileinput import sys import re pat = re.compile(r"((?:=|:|^)\s*load (?:atomic )?(?:volatile )?(.*?))(| addrspace\(\d+\) *)\*($| *(?:%|@|null|undef|blockaddress|getelementptr|addrspacecast|bitcast|inttoptr|\[\[[a-zA-Z]|\{\{).*$)") for line in sys.stdin: sys.stdout.write(re.sub(pat, r"\1, \2\3*\4", line)) Reviewers: rafael, dexonsmith, grosser Differential Revision: http://reviews.llvm.org/D7649 llvm-svn: 230794
2015-02-27 22:17:42 +01:00
%0 = load i32, i32* %a.addr, align 4
LowerSwitch: track bounding range for the condition tree. When LowerSwitch transforms a switch instruction into a tree of ifs it is actually performing a binary search into the various case ranges, to see if the current value falls into one cases range of values. So, if we have a program with something like this: switch (a) { case 0: do0(); break; case 1: do1(); break; case 2: do2(); break; default: break; } the code produced is something like this: if (a < 1) { if (a == 0) { do0(); } } else { if (a < 2) { if (a == 1) { do1(); } } else { if (a == 2) { do2(); } } } This code is inefficient because the check (a == 1) to execute do1() is not needed. The reason is that because we already checked that (a >= 1) initially by checking that also (a < 2) we basically already inferred that (a == 1) without the need of an extra basic block spawned to check if actually (a == 1). The patch addresses this problem by keeping track of already checked bounds in the LowerSwitch algorithm, so that when the time arrives to produce a Leaf Block that checks the equality with the case value / range the algorithm can decide if that block is really needed depending on the already checked bounds . For example, the above with "a = 1" would work like this: the bounds start as LB: NONE , UB: NONE as (a < 1) is emitted the bounds for the else path become LB: 1 UB: NONE. This happens because by failing the test (a < 1) we know that the value "a" cannot be smaller than 1 if we enter the else branch. After the emitting the check (a < 2) the bounds in the if branch become LB: 1 UB: 1. This is because by checking that "a" is smaller than 2 then the upper bound becomes 2 - 1 = 1. When it is time to emit the leaf block for "case 1:" we notice that 1 can be squeezed exactly in between the LB and UB, which means that if we arrived to that block there is no need to emit a block that checks if (a == 1). Patch by: Marcello Maggioni <hayarms@gmail.com> llvm-svn: 211038
2014-06-16 18:55:20 +02:00
switch i32 %0, label %sw.default [
i32 0, label %sw.bb
i32 1, label %sw.bb1
i32 2, label %sw.bb2
]
sw.bb:
ret i32 12
sw.bb1:
ret i32 4
sw.bb2:
ret i32 2
sw.default:
ret i32 9
}
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