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Diffstat (limited to 'gcc/doc/tree-ssa.texi')
| -rw-r--r-- | gcc/doc/tree-ssa.texi | 39 |
1 files changed, 38 insertions, 1 deletions
diff --git a/gcc/doc/tree-ssa.texi b/gcc/doc/tree-ssa.texi index 6996e83..4679912 100644 --- a/gcc/doc/tree-ssa.texi +++ b/gcc/doc/tree-ssa.texi @@ -1208,9 +1208,46 @@ hooks to execute custom code at various points during traversal: @cindex flow-sensitive alias analysis @cindex flow-insensitive alias analysis -Alias analysis proceeds in 3 main phases: +Alias analysis proceeds in 4 main phases: @enumerate +@item Structural alias analysis. + +This phase walks the types for structure variables, and determines which +of the fields can overlap using offset and size of each field. For each +field, a ``subvariable'' called a ``Structure field tag'' (SFT)@ is +created, which represents that field as a separate variable. All +accesses that could possibly overlap with a given field will have +virtual operands for the SFT of that field. + +@smallexample +struct foo +@{ + int a; + int b; +@} +struct foo temp; +int bar (void) +@{ + int tmp1, tmp2, tmp3; + SFT.0_2 = V_MUST_DEF <SFT.0_1> + temp.a = 5; + SFT.1_4 = V_MUST_DEF <SFT.1_3> + temp.b = 6; + + VUSE <SFT.1_4> + tmp1_5 = temp.b; + VUSE <SFT.0_2> + tmp2_6 = temp.a; + + tmp3_7 = tmp1_5 + tmp2_6; + return tmp3_7; +@} +@end smallexample + +If you copy the type tag for a variable for some reason, you probably +also want to copy the subvariables for that variable. + @item Points-to and escape analysis. This phase walks the use-def chains in the SSA web looking for |