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middle-end: Correct calculation of mul_widen_cost and mul_highpart_cost.

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This patch fixes a subtle bug in the depths of GCC's synth_mult,
where the middle-end queries whether (how well) the target supports
widening and highpart multiplications by calling targetm.rtx_costs.
The code in init_expmed and init_expmed_one_mode iterates over various
RTL patterns querying the cost of each.  To avoid generating & garbage
collecting too much junk, it reuses the same RTL over and over, but
adjusting the modes between each call.

Alas this reuse of state is a little fragile, and at some point a
change to init_expmed_one_conv has resulted in the state (mode of
a register) being changed, but not reset before being used again.

Using the old software engineering/defensive programming maxim of
"why fix a bug just once, if it can be fixed in multiple places",
this patch both restores the original value in init_expmed_one_conv,
and also sets it to the expected value in init_expmed_one_mode.
This should hopefully signal the need to be careful of invariants for
anyone modifying this code in future.

2020-08-09  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* expmed.c (init_expmed_one_conv): Restore all->reg's mode.
	(init_expmed_one_mode): Set all->reg to desired mode.

gcc/testsuite/ChangeLog
	PR target/71321
	* gcc.target/i386/pr71321.c: Check that the code doesn't use
	the 4B zero displacement lea, not that it uses lea.
This commit is contained in:
Roger Sayle 2020-08-09 23:14:58 +01:00
parent 94d4f4387d
commit 71197a5d13
2 changed files with 4 additions and 2 deletions
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3
gcc/expmed.c
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@ -155,6 +155,8 @@ init_expmed_one_conv (struct init_expmed_rtl *all, scalar_int_mode to_mode,
PUT_MODE (all->reg, from_mode);
set_convert_cost (to_mode, from_mode, speed,
set_src_cost (which, to_mode, speed));
/* Restore all->reg's mode. */
PUT_MODE (all->reg, to_mode);
}
static void
@ -229,6 +231,7 @@ init_expmed_one_mode (struct init_expmed_rtl *all,
if (GET_MODE_CLASS (int_mode_to) == MODE_INT
&& GET_MODE_WIDER_MODE (int_mode_to).exists (&wider_mode))
{
PUT_MODE (all->reg, mode);
PUT_MODE (all->zext, wider_mode);
PUT_MODE (all->wide_mult, wider_mode);
PUT_MODE (all->wide_lshr, wider_mode);

3
gcc/testsuite/gcc.target/i386/pr71321.c
View file

@ -12,5 +12,4 @@ unsigned cvt_to_2digit_ascii(uint8_t i)
{
return cvt_to_2digit(i, 10) + 0x0a3030;
}
/* { dg-final { scan-assembler-times "lea.\t\\(%\[0-9a-z\]+,%\[0-9a-z\]+,4" 3 } } */
/* { dg-final { scan-assembler-times "lea.\t\\(%\[0-9a-z\]+,%\[0-9a-z\]+,8" 1 } } */
/* { dg-final { scan-assembler-not "lea.*0" } } */