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Disable FMA by default. Use -Ofma or jit.opt.start("+fma") to enable.

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See the discussion in #918 for the rationale.
This commit is contained in:
Mike Pall 2022-12-07 18:38:22 +01:00
parent 7d5d4a1b1a
commit de2e1ca9d3
7 changed files with 35 additions and 6 deletions
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8
doc/running.html
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@ -220,6 +220,12 @@ mix the three forms, but note that setting an optimization level
overrides all earlier flags.
</p>
<p>
Note that <tt>-Ofma</tt> is not enabled by default at any level,
because it affects floating-point result accuracy. Only enable this,
if you fully understand the trade-offs of FMA for performance (higher),
determinism (lower) and numerical accuracy (higher).
</p>
<p>
Here are the available flags and at what optimization levels they
are enabled:
</p>
@ -251,6 +257,8 @@ are enabled:
<td class="flag_name">sink</td><td class="flag_level">&nbsp;</td><td class="flag_level">&nbsp;</td><td class="flag_level">&bull;</td><td class="flag_desc">Allocation/Store Sinking</td></tr>
<tr class="even">
<td class="flag_name">fuse</td><td class="flag_level">&nbsp;</td><td class="flag_level">&nbsp;</td><td class="flag_level">&bull;</td><td class="flag_desc">Fusion of operands into instructions</td></tr>
<tr class="odd">
<td class="flag_name">fma </td><td class="flag_level">&nbsp;</td><td class="flag_level">&nbsp;</td><td class="flag_level">&nbsp;</td><td class="flag_desc">Fused multiply-add</td></tr>
</table>
<p>
Here are the parameters and their default settings:

6
src/lj_asm_arm.h
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@ -313,7 +313,11 @@ static void asm_fusexref(ASMState *as, ARMIns ai, Reg rd, IRRef ref,
}
#if !LJ_SOFTFP
/* Fuse to multiply-add/sub instruction. */
/*
** Fuse to multiply-add/sub instruction.
** VMLA rounds twice (UMA, not FMA) -- no need to check for JIT_F_OPT_FMA.
** VFMA needs VFPv4, which is uncommon on the remaining ARM32 targets.
*/
static int asm_fusemadd(ASMState *as, IRIns *ir, ARMIns ai, ARMIns air)
{
IRRef lref = ir->op1, rref = ir->op2;

3
src/lj_asm_arm64.h
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@ -337,7 +337,8 @@ static int asm_fusemadd(ASMState *as, IRIns *ir, A64Ins ai, A64Ins air)
{
IRRef lref = ir->op1, rref = ir->op2;
IRIns *irm;
if (lref != rref &&
if ((as->flags & JIT_F_OPT_FMA) &&
lref != rref &&
((mayfuse(as, lref) && (irm = IR(lref), irm->o == IR_MUL) &&
ra_noreg(irm->r)) ||
(mayfuse(as, rref) && (irm = IR(rref), irm->o == IR_MUL) &&

3
src/lj_asm_ppc.h
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@ -235,7 +235,8 @@ static int asm_fusemadd(ASMState *as, IRIns *ir, PPCIns pi, PPCIns pir)
{
IRRef lref = ir->op1, rref = ir->op2;
IRIns *irm;
if (lref != rref &&
if ((as->flags & JIT_F_OPT_FMA) &&
lref != rref &&
((mayfuse(as, lref) && (irm = IR(lref), irm->o == IR_MUL) &&
ra_noreg(irm->r)) ||
(mayfuse(as, rref) && (irm = IR(rref), irm->o == IR_MUL) &&

4
src/lj_jit.h
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@ -87,10 +87,11 @@
#define JIT_F_OPT_ABC (JIT_F_OPT << 7)
#define JIT_F_OPT_SINK (JIT_F_OPT << 8)
#define JIT_F_OPT_FUSE (JIT_F_OPT << 9)
#define JIT_F_OPT_FMA (JIT_F_OPT << 10)
/* Optimizations names for -O. Must match the order above. */
#define JIT_F_OPTSTRING \
"\4fold\3cse\3dce\3fwd\3dse\6narrow\4loop\3abc\4sink\4fuse"
"\4fold\3cse\3dce\3fwd\3dse\6narrow\4loop\3abc\4sink\4fuse\3fma"
/* Optimization levels set a fixed combination of flags. */
#define JIT_F_OPT_0 0
@ -99,6 +100,7 @@
#define JIT_F_OPT_3 (JIT_F_OPT_2|\
JIT_F_OPT_FWD|JIT_F_OPT_DSE|JIT_F_OPT_ABC|JIT_F_OPT_SINK|JIT_F_OPT_FUSE)
#define JIT_F_OPT_DEFAULT JIT_F_OPT_3
/* Note: FMA is not set by default. */
/* -- JIT engine parameters ----------------------------------------------- */

13
src/lj_vmmath.c
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@ -36,6 +36,17 @@ LJ_FUNCA double lj_wrap_fmod(double x, double y) { return fmod(x, y); }
/* -- Helper functions ---------------------------------------------------- */
/* Required to prevent the C compiler from applying FMA optimizations.
**
** Yes, there's -ffp-contract and the FP_CONTRACT pragma ... in theory.
** But the current state of C compilers is a mess in this regard.
** Also, this function is not performance sensitive at all.
*/
LJ_NOINLINE static double lj_vm_floormul(double x, double y)
{
return lj_vm_floor(x / y) * y;
}
double lj_vm_foldarith(double x, double y, int op)
{
switch (op) {
@ -43,7 +54,7 @@ double lj_vm_foldarith(double x, double y, int op)
case IR_SUB - IR_ADD: return x-y; break;
case IR_MUL - IR_ADD: return x*y; break;
case IR_DIV - IR_ADD: return x/y; break;
case IR_MOD - IR_ADD: return x-lj_vm_floor(x/y)*y; break;
case IR_MOD - IR_ADD: return x-lj_vm_floormul(x, y); break;
case IR_POW - IR_ADD: return pow(x, y); break;
case IR_NEG - IR_ADD: return -x; break;
case IR_ABS - IR_ADD: return fabs(x); break;

4
src/vm_arm64.dasc
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@ -2636,7 +2636,9 @@ static void build_ins(BuildCtx *ctx, BCOp op, int defop)
|.macro ins_arithmod, res, reg1, reg2
| fdiv d2, reg1, reg2
| frintm d2, d2
| fmsub res, d2, reg2, reg1
| // Cannot use fmsub, because FMA is not enabled by default.
| fmul d2, d2, reg2
| fsub res, reg1, d2
|.endmacro
|
|.macro ins_arithdn, intins, fpins