i386: Correct insn_cost of movabsq.

This single line patch fixes a strange quirk/glitch in i386's rtx_costs,
which considers an instruction loading a 64-bit constant to be significantly
cheaper than loading a 32-bit (or smaller) constant.

Consider the two functions:
unsigned long long foo() { return 0x0123456789abcdefULL; }
unsigned int bar() { return 10; }

and the corresponding lines from combine's dump file:
  insn_cost 1 for #: r98:DI=0x123456789abcdef
  insn_cost 4 for #: ax:SI=0xa

The same issue can be seen in -dP assembler output.
  movabsq $81985529216486895, %rax    # 5  [c=1 l=10]  *movdi_internal/4

The problem is that pattern_costs interpretation of rtx_costs contains
"return cost > 0 ? cost : COSTS_N_INSNS (1)" where a zero value (for
example a register or small immediate constant) is considered special,
and equivalent to a single instruction, but all other values are treated
as verbatim.  Hence to x86_64's 10-byte long movabsq instruction slightly
more expensive than a simple constant, rtx_costs needs to return
COSTS_N_INSNS(1)+1 and not 1.  With this change, the insn_cost of
movabsq is the intended value 5:
  insn_cost 5 for #: r98:DI=0x123456789abcdef
and
  movabsq $81985529216486895, %rax    # 5  [c=5 l=10]  *movdi_internal/4

2024-05-22  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* config/i386/i386.cc (ix86_rtx_costs) <case CONST_INT>:
	A CONST_INT that isn't x86_64_immediate_operand requires an extra
	(expensive) movabsq insn to load, so return COSTS_N_INSNS (1) + 1.

diff --git a/gcc/config/i386/i386.cc b/gcc/config/i386/i386.cc
index 69cd4ae05a74919b49cb1f7122bc99929fdaee9f..3e2a3a194f1c812dca22f71d419b285d30f120e0 100644 (file)
--- a/gcc/config/i386/i386.cc
+++ b/gcc/config/i386/i386.cc
@@ -21562,7 +21562,8 @@ ix86_rtx_costs (rtx x, machine_mode mode, int outer_code_i, int opno,
       if (x86_64_immediate_operand (x, VOIDmode))
        *total = 0;
      else
-       *total = 1;
+       /* movabsq is slightly more expensive than a simple instruction. */
+       *total = COSTS_N_INSNS (1) + 1;
       return true;
 
     case CONST_DOUBLE: