[PATCH] peephole optimization in switch table lookup: reuse the guarding table comparison if possible

[Erik Eckstein]

Hi hans,

This optimization tries to reuse the generated compare instruction, if there is a comparison agains the default value after the switch.

Example:

switch (x) {
 case 0: r = 10; break;
 case 1: r = 11; break;
 ...
 default: r = 0; break; // 0 does not appear in any case value.
}
if (r == 0) {
 do_something;
}

transforms to:

cond = x < table_size;
if (cond) {
 r = table[x];
} else {
 r = 0;
}
if (cond) {
  do_something;
}

Jump threading can then eliminate the second if (cond):

if (x < table_size) {
 r = table[x];
} else {
 r = 0;
 do_something;
}

http://reviews.llvm.org/D6423

Files:
  lib/Transforms/Utils/SimplifyCFG.cpp
  test/Transforms/SimplifyCFG/X86/switch_to_lookup_table.ll
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