From b4c969723c1e61dbdaadc61b0b5fd5393e9ff76e Mon Sep 17 00:00:00 2001 From: Richard Sandiford Date: Sat, 6 Jan 2018 11:07:53 +0000 Subject: [PATCH] Tighten LRA cycling check LRA has code to try to prevent cycling, by avoiding reloads that look too similar to the instruction being reloaded. E.g. if we have a R<-C move for some constant C, reloading the source with another R<-C move is unlikely to be a good idea. However, this safeguard unnecessarily triggered in tests like the one in the patch. We started with instructions like: (insn 12 9 13 5 (set (reg:DI 0 x0) (reg/f:DI 459)) "reg-alloc-1.c":18 47 {*movdi_aarch64} (expr_list:REG_EQUAL (symbol_ref:DI ("x00") [flags 0xc0] ) (nil))) where r459 didn't get allocated a register and is equivalent to constant x00. LRA would then handle it like this: Changing pseudo 459 in operand 1 of insn 12 on equiv `x00' 1 Non-pseudo reload: reject+=2 1 Non input pseudo reload: reject++ -------> Cycle danger: overall += LRA_MAX_REJECT alt=0,overall=609,losers=1,rld_nregs=1 [...] alt=13,overall=9,losers=1,rld_nregs=1 [...] Choosing alt 13 in insn 12: (0) r (1) w {*movdi_aarch64} In other words, to avoid loading the constant x00 into another GPR, LRA decided instead to move it into a floating-point register, then move that floating-point register into x0: Creating newreg=630, assigning class FP_REGS to r630 Set class ALL_REGS for r631 12: x0:DI=r630:DI REG_EQUAL `x00' Inserting insn reload before: 815: r631:DI=high(`x00') 816: r630:DI=r631:DI+low(`x00') REG_EQUAL `x00' That's inefficient and doesn't really help to resolve a cycling problem, since the r630 destination of 816 needs to be reloaded into a GPR anyway. The cycling check already had an exception for source values that are the result of an elimination. This patch extends it to include the result of equivalence substitution. 2018-01-06 Richard Sandiford gcc/ * lra-constraints.c (process_alt_operands): Test for the equivalence substitutions when detecting a possible reload cycle. gcc/testsuite/ * gcc.target/aarch64/reg-alloc-1.c: New test. From-SVN: r256312 --- gcc/ChangeLog | 5 ++++ gcc/lra-constraints.c | 7 ++++- gcc/testsuite/ChangeLog | 4 +++ .../gcc.target/aarch64/reg-alloc-1.c | 26 +++++++++++++++++++ 4 files changed, 41 insertions(+), 1 deletion(-) create mode 100644 gcc/testsuite/gcc.target/aarch64/reg-alloc-1.c diff --git a/gcc/ChangeLog b/gcc/ChangeLog index 59f25abe8b5..767d7458bb3 100644 --- a/gcc/ChangeLog +++ b/gcc/ChangeLog @@ -1,3 +1,8 @@ +2018-01-06 Richard Sandiford + + * lra-constraints.c (process_alt_operands): Test for the equivalence + substitutions when detecting a possible reload cycle. + 2018-01-06 Jakub Jelinek PR debug/83480 diff --git a/gcc/lra-constraints.c b/gcc/lra-constraints.c index 4d307af7011..3af31999ad8 100644 --- a/gcc/lra-constraints.c +++ b/gcc/lra-constraints.c @@ -2866,7 +2866,12 @@ process_alt_operands (int only_alternative) /* If it is a result of recent elimination in move insn we can transform it into an add still by using this alternative. */ - && GET_CODE (no_subreg_reg_operand[1]) != PLUS))) + && GET_CODE (no_subreg_reg_operand[1]) != PLUS + /* Likewise if the source has been replaced with an + equivalent value. This only happens once -- the reload + will use the equivalent value instead of the register it + replaces -- so there should be no danger of cycling. */ + && !equiv_substition_p[1]))) { /* We have a move insn and a new reload insn will be similar to the current insn. We should avoid such situation as diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog index 4c2b209f33a..2b9d7aeec2d 100644 --- a/gcc/testsuite/ChangeLog +++ b/gcc/testsuite/ChangeLog @@ -1,3 +1,7 @@ +2018-01-06 Richard Sandiford + + * gcc.target/aarch64/reg-alloc-1.c: New test. + 2018-01-06 Janne Blomqvist PR fortran/50892 diff --git a/gcc/testsuite/gcc.target/aarch64/reg-alloc-1.c b/gcc/testsuite/gcc.target/aarch64/reg-alloc-1.c new file mode 100644 index 00000000000..28a2a99db43 --- /dev/null +++ b/gcc/testsuite/gcc.target/aarch64/reg-alloc-1.c @@ -0,0 +1,26 @@ +/* { dg-do compile } */ +/* { dg-options "-O2 -fdump-rtl-reload-details" } */ + +#define R1(X, Y) X (Y##0) X (Y##1) X (Y##2) X (Y##3) \ + X (Y##4) X (Y##5) X (Y##6) X (Y##7) +#define R2(X) R1 (X, 0) R1 (X, 1) R1 (X, 2) R1 (X, 3) + +#define DEFINE(N) extern int x##N; +R2 (DEFINE) + +void b1 (int *); + +void +foo (int n) +{ + for (int i = 0; i < n; ++i) + { +#define CALL(N) b1 (&x##N); + R2 (CALL); + R2 (CALL); + } +#define INC(N) x##N += 1; + R2 (INC); +} + +/* { dg-final { scan-rtl-dump-not "DI 32 v0" "reload" } } */ -- 2.30.2