From: Xionghu Luo <luo...@linux.ibm.com> This "subtract/extend/add" existed for a long time and still annoying us (PR37451, PR61837) when converting from 32bits to 64bits, as the ctr register is used as 64bits on powerpc64, Andraw Pinski had a patch but caused some issue and reverted by Joseph S. Myers(PR37451, PR37782).
Andraw: http://gcc.gnu.org/ml/gcc-patches/2008-09/msg01070.html http://gcc.gnu.org/ml/gcc-patches/2008-10/msg01321.html Joseph: https://gcc.gnu.org/legacy-ml/gcc-patches/2011-11/msg02405.html However, the doloop code improved a lot since so many years passed, gcc.c-torture/execute/doloop-1.c is no longer a simple loop with constant desc->niter_expr since r125:SI#0 is not SImode, so it is not a valid doloop and no transform done in doloop again. Thus we can do the simplification from "subtract/extend/add" to only extend as the condition in doloop will never be false based on loop ch's optimization. What's more, this patch is slightly different with Andrw's implementation, the check of ZERO_EXT and SImode will guard the count won't be changed from char/short caused cases not time out on slow platforms before. Any comments? Thanks. doloop-1.c.257r.loop2_doloop ... 12: [r129:DI]=r123:SI REG_DEAD r129:DI REG_DEAD r123:SI 13: r125:SI=r120:DI#0-0x1 REG_DEAD r120:DI 14: r120:DI=zero_extend(r125:SI#0) REG_DEAD r125:SI 16: r126:CC=cmp(r120:DI,0) 17: pc={(r126:CC!=0)?L43:pc} REG_DEAD r126:CC ... Bootstrap and regression tested pass on Power8-LE. gcc/ChangeLog 2020-04-15 Xiong Hu Luo <luo...@linux.ibm.com> PR rtl-optimization/37451, PR target/61837 loop-doloop.c (doloop_modify): Simplify (add -1; zero_ext; add +1) to zero_ext. --- gcc/loop-doloop.c | 26 +++++++++++++++++++++++++- 1 file changed, 25 insertions(+), 1 deletion(-) diff --git a/gcc/loop-doloop.c b/gcc/loop-doloop.c index db6a014e43d..9f967fa3a0b 100644 --- a/gcc/loop-doloop.c +++ b/gcc/loop-doloop.c @@ -477,7 +477,31 @@ doloop_modify (class loop *loop, class niter_desc *desc, } if (increment_count) - count = simplify_gen_binary (PLUS, mode, count, const1_rtx); + { + /* Fold (add -1; zero_ext; add +1) operations to zero_ext based on addop0 + is never zero, as gimple pass loop ch will do optimization to simplify + the loop to NO loop for loop condition is false. */ + bool simplify_zext = false; + rtx extop0 = XEXP (count, 0); + if (mode == E_DImode + && GET_CODE (count) == ZERO_EXTEND + && GET_CODE (extop0) == PLUS) + { + rtx addop0 = XEXP (extop0, 0); + rtx addop1 = XEXP (extop0, 1); + if (CONST_SCALAR_INT_P (addop1) + && GET_MODE (addop0) == E_SImode + && addop1 == GEN_INT (-1)) + { + count = simplify_gen_unary (ZERO_EXTEND, mode, addop0, + GET_MODE (addop0)); + simplify_zext = true; + } + } + + if (!simplify_zext) + count = simplify_gen_binary (PLUS, mode, count, const1_rtx); + } /* Insert initialization of the count register into the loop header. */ start_sequence (); -- 2.21.0.777.g83232e3864
