The following fixes the oldest bug in my list of assigned PRs, namely combining A / CST / CST' to A / (CST * CST'). extract_muldiv does this in fold-const.c (but it fails to optimize to zero when CST * CST' overflows).
Bootstrapped and tested on x86_64-unknown-linux-gnu, applied to trunk. Richard. 2014-12-01 Richard Biener <rguent...@suse.de> PR tree-optimization/15346 * Makefile.in (gimple-match.o-warn): Remove -Wno-unused-parameter, add -Wno-unused-but-set-variable. * match.pd: Combine two successive divisions. * gcc.dg/tree-ssa/forwprop-32.c: New testcase. Index: gcc/match.pd =================================================================== --- gcc/match.pd (revision 218140) +++ gcc/match.pd (working copy) @@ -129,6 +129,19 @@ (define_operator_list inverted_tcc_compa && TYPE_UNSIGNED (type)) (trunc_div @0 @1))) +/* Combine two successive divisions. */ +(for div (trunc_div ceil_div floor_div round_div exact_div) + (simplify + (div (div @0 INTEGER_CST@1) INTEGER_CST@2) + (with { + bool overflow_p; + wide_int mul = wi::mul (@1, @2, TYPE_SIGN (type), &overflow_p); + } + (if (!overflow_p) + (div @0 { wide_int_to_tree (type, mul); })) + (if (overflow_p) + { build_zero_cst (type); })))) + /* Optimize A / A to 1.0 if we don't care about NaNs or Infinities. */ (simplify Index: gcc/testsuite/gcc.dg/tree-ssa/forwprop-32.c =================================================================== --- gcc/testsuite/gcc.dg/tree-ssa/forwprop-32.c (revision 0) +++ gcc/testsuite/gcc.dg/tree-ssa/forwprop-32.c (working copy) @@ -0,0 +1,18 @@ +/* { dg-do compile } */ +/* { dg-options "-O -fdump-tree-forwprop1 -fdump-tree-ccp1" } */ + +int foo (int x) +{ + int tem = x / 3; + return tem / 5; +} +int bar (int x) +{ + int tem = x / 3; + return tem / (__INT_MAX__ / 2); +} + +/* { dg-final { scan-tree-dump "x_.\\(D\\) / 15" "forwprop1" } } */ +/* { dg-final { scan-tree-dump "return 0;" "ccp1" } } */ +/* { dg-final { cleanup-tree-dump "forwprop1" } } */ +/* { dg-final { cleanup-tree-dump "ccp1" } } */ Index: gcc/Makefile.in =================================================================== --- gcc/Makefile.in (revision 218142) +++ gcc/Makefile.in (working copy) @@ -209,8 +209,8 @@ gengtype-lex.o-warn = -Wno-error libgcov-util.o-warn = -Wno-error libgcov-driver-tool.o-warn = -Wno-error libgcov-merge-tool.o-warn = -Wno-error -gimple-match.o-warn = -Wno-unused-variable -Wno-unused-parameter -generic-match.o-warn = -Wno-unused-variable -Wno-unused-parameter +gimple-match.o-warn = -Wno-unused-variable -Wno-unused-but-set-variable +generic-match.o-warn = -Wno-unused-variable -Wno-unused-but-set-variable # All warnings have to be shut off in stage1 if the compiler used then # isn't gcc; configure determines that. WARN_CFLAGS will be either