+CC gcc-patches
On 5/30/25 14:04, Vineet Gupta wrote:
> Hi Jeff, Richard
>
> As part of RISC-V FRM mode switching improvements, I'm running into a behavior
> in late_combine2 where it is eliminating FRM save/restores when it is desired
> to
> keep them.
>
> I'm pasting snippet of RTL dumps, could you please see if u is anything
> jumping
> out from this limited info.
> In RISC-V backend, FRM is specified as a global_reg and inline asm is the
> only
> way for users to achieve the global fesetround() like semantics.
>
> src
>
> /* -march=rv64gcv -mabi=lp64d -O3 -ftree-vectorize */
> #pragma riscv intrinsic "vector"
> typedef long unsigned int size_t;
>
> static void
> set_frm (int frm)
> {
> __asm__ volatile ( "fsrm %0" : :"r"(frm) : "frm");
> }
>
> vfloat32m1_t __attribute__ ((noinline))
> test_float_point_frm_run_1 (vfloat32m1_t op1, vfloat32m1_t op2, size_t vl)
> {
> vfloat32m1_t result;
> /* global mode set */
> set_frm (0);
> /* intrinsic for set mode 1 should be local and restored back to global > 0
> upon return */
> result = __riscv_vfadd_vv_f32m1_rm (op1, result, 1, vl);
> return result;
> }
>
>
> RTL dump
>
> mode-sw
>
> (insn 9 8 18 2 (parallel [
> (asm_operands/v ("fsrm %0") ("") 0 [
> (reg:SI 139)
> ]
> [
> (asm_input:SI ("r") frm-run-1.c:33)
> ]
> [] frm-run-1.c:33)
> (clobber (reg:V4096QI 69 frm))
> ]) "frm-run-1.c":33:3 -1
> (expr_list:REG_DEAD (reg:SI 139)
> (nil)))
>
> (insn 27 10 28 2 (set (reg:SI 144)
> (reg:SI 69 frm)) "frm-run-1.c":43:1 -1
> (nil))
> (insn 28 27 14 2 (set (reg:SI 69 frm)
> (const_int 1 [0x1])) "frm-run-1.c":43:1 -1
> (nil))
>
> (insn 29 14 24 2 (set (reg:SI 69 frm)
> (reg:SI 144)) -1
> (nil))
>
> late-combine2
>
> trying to combine definition of r15 in:
> 27: a5:SI=frm:SI
> into:
> 29: frm:SI=a5:SI
> instruction becomes a no-op:
> (set (reg:SI 69 frm)
> (reg:SI 69 frm))
> original cost = 4 + 4 (weighted: 8.000000), replacement cost = nop;
> keeping
> replacement
> rescanning insn with uid = 29.
> updating insn 29 in-place
> verify found no changes in insn with uid = 29.
> deleting insn 27
> deleting insn with uid = 27
>
>
> If I comment out the inline asm - it can no longer combine, elimination
> doesn't
> happen with expected outcome.
>
> trying to combine definition of r15 in:
> 25: a5:SI=frm:SI
> into:
> 27: frm:SI=a5:SI
> -- cannot satisfy all definitions and uses in insn 27
>
>
> Thx,
> -Vineet
>
> P.S. The RV changes needed to trigger this are still in-flight, but available
> here [1] if needed
>
> [1] https://github.com/vineetgarc/gcc/tree/frm-mode-switch/upstream-v2
>
