On Mon, Jun 20, 2016 at 12:46 PM, Richard Sandiford <rdsandif...@googlemail.com> wrote: > Uros Bizjak <ubiz...@gmail.com> writes: >> On Mon, Jun 20, 2016 at 9:19 PM, H.J. Lu <hjl.to...@gmail.com> wrote: >>> On Mon, Jun 20, 2016 at 12:13 PM, Uros Bizjak <ubiz...@gmail.com> wrote: >>>> On Mon, Jun 20, 2016 at 7:05 PM, H.J. Lu <hongjiu...@intel.com> wrote: >>>>> Hi, >>>>> >>>>> This patch implements the alternate code sequence recommended in >>>>> >>>>> https://groups.google.com/forum/#!topic/x86-64-abi/de5_KnLHxtI >>>>> >>>>> to load external function address via GOT slot with >>>>> >>>>> movq func@GOTPCREL(%rip), %rax >>>>> >>>>> so that linker won't create an PLT entry for extern function >>>>> address. >>>>> >>>>> Tested on x86-64. OK for trunk? >>>> >>>>> + else if (ix86_force_load_from_GOT_p (op1)) >>>>> + { >>>>> + /* Load the external function address via the GOT slot to >>>>> + avoid PLT. */ >>>>> + op1 = gen_rtx_UNSPEC (Pmode, gen_rtvec (1, op1), >>>>> + (TARGET_64BIT >>>>> + ? UNSPEC_GOTPCREL >>>>> + : UNSPEC_GOT)); >>>>> + op1 = gen_rtx_CONST (Pmode, op1); >>>>> + op1 = gen_const_mem (Pmode, op1); >>>>> + /* This symbol must be referenced via a load from the Global >>>>> + Offset Table. */ >>>>> + set_mem_alias_set (op1, ix86_GOT_alias_set ()); >>>>> + op1 = convert_to_mode (mode, op1, 1); >>>>> + op1 = force_reg (mode, op1); >>>>> + emit_insn (gen_rtx_SET (op0, op1)); >>>>> + /* Generate a CLOBBER so that there will be no REG_EQUAL note >>>>> + on the last insn to prevent cse and fwprop from replacing >>>>> + a GOT load with a constant. */ >>>>> + rtx tmp = gen_reg_rtx (Pmode); >>>>> + emit_clobber (tmp); >>>>> + return; >>>> >>>> Jeff, is this the recommended way to prevent CSE, as far as RTL >>>> infrastructure is concerned? I didn't find any example of this >>>> approach with other targets. >>>> >>> >>> FWIW, the similar approach is used in ix86_expand_vector_move_misalign, >>> ix86_expand_convert_uns_didf_sse and ix86_expand_vector_init_general >>> as well as other targets: >>> >>> frv/frv.c: emit_clobber (op0); >>> frv/frv.c: emit_clobber (op1); >>> im32c/m32c.c: /* emit_clobber (gen_rtx_REG (HImode, R0L_REGNO)); */ >>> s390/s390.c: emit_clobber (addr); >>> s390/s390.md: emit_clobber (reg0); >>> s390/s390.md: emit_clobber (reg1); >>> s390/s390.md: emit_clobber (reg0); >>> s390/s390.md: emit_clobber (reg0); >>> s390/s390.md: emit_clobber (reg1); >> >> These usages mark the whole register as being "clobbered" >> (=undefined), before only a part of register is written, e.g.: >> >> emit_clobber (int_xmm); >> emit_move_insn (gen_lowpart (DImode, int_xmm), input); >> >> They aren't used to prevent unwanted CSE. > > Since it's being called in the move expander, I thought the normal > way of preventing the constant being rematerialised would be to reject > it in the move define_insn predicates. > > FWIW, I agree that using a clobber for this is going to be fragile. >
Here is the alternative from clobber. -- H.J. -- diff --git a/gcc/config/i386/i386.md b/gcc/config/i386/i386.md index a68983c..79999df 100644 --- a/gcc/config/i386/i386.md +++ b/gcc/config/i386/i386.md @@ -2347,7 +2347,7 @@ (define_insn "*movsi_internal" [(set (match_operand:SI 0 "nonimmediate_operand" "=r,m ,*y,*y,?rm,?*y,*v,*v,*v,m ,?r ,?r,?*Yi,*k ,*rm") - (match_operand:SI 1 "general_operand" + (match_operand:SI 1 "ix86_general_operand" "g ,re,C ,*y,*y ,rm ,C ,*v,m ,*v,*Yj,*v,r ,*krm,*k"))] "!(MEM_P (operands[0]) && MEM_P (operands[1]))" { @@ -2564,7 +2564,7 @@ (define_insn "*movqi_internal" [(set (match_operand:QI 0 "nonimmediate_operand" "=q,q ,q ,r,r ,?r,m ,k,k,r ,m,k") - (match_operand:QI 1 "general_operand" + (match_operand:QI 1 "ix86_general_operand" "q ,qn,qm,q,rn,qm,qn,r ,k,k,k,m"))] "!(MEM_P (operands[0]) && MEM_P (operands[1]))" { diff --git a/gcc/config/i386/predicates.md b/gcc/config/i386/predicates.md index 06a0002..a471deb 100644 --- a/gcc/config/i386/predicates.md +++ b/gcc/config/i386/predicates.md @@ -367,6 +367,12 @@ } }) +;; Return true if OP is general operand representable on ix86 +(define_predicate "ix86_general_operand" + (and (match_operand 0 "general_operand") + (ior (not (match_code "symbol_ref")) + (match_test "!ix86_force_load_from_GOT_p (op)")))) + ;; Return true if size of VALUE can be stored in a sign ;; extended immediate field. (define_predicate "x86_64_immediate_size_operand" @@ -1036,6 +1042,9 @@ struct ix86_address parts; int ok; + if (ix86_force_load_from_GOT_p (op)) + return false; + if (!CONST_INT_P (op) && mode != VOIDmode && GET_MODE (op) != mode)