RISC-V: fix __builtin_round clobbering FP exceptions flags [PR121534]

__builtin_round() fails to save/restore FP exception flags around the FP
compare insn which can potentially clobber the same.

Worth noting that the fflags restore bracketing is slightly different
than the glibc implementation. Both FLT and FCVT can potentially clobber
fflags. gcc generates below where even if branch is not taken and FCVT
is not executed, FLT still executed. Thus FSFLAGS is placed AFTER the
label 'L3'. glibc implementation FLT can't clobber due to early NaN check,
so FSFLAGS can be moved under the branch, before the label.

| convert_float_to_float_round
| ...
|   frflags	a5
|   fabs.s	fa5,fa0
|   flt.s	a4,fa5,fa4    <--- can clobber fflags
|   beq	a4,zero,.L3
|     fcvt.w.s a4,fa0,rmm     <--- also
|     fcvt.s.w	fa5,a4
|     fsgnj.s	fa0,fa5,fa0
| .L3:
|    fsflags	a5            <-- both code paths

Fixes: f652a35877e3 ("This is almost exclusively Jivan's work....")
	PR target/121534

gcc/ChangeLog:

	* config/riscv/riscv.md (round_pattern): save/restore fflags.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/vls/math-nearbyint-1.c: Adjust
	scan pattern for additional instances of frflags/fsrflags.

Signed-off-by: Vineet Gupta <vineetg@rivosinc.com>

diff --git a/gcc/config/riscv/riscv.md b/gcc/config/riscv/riscv.md
index 3b779e2c060fb0e952c5f23dcba8f57c85e0451a..a02f3d2c279d41de26893d42f71edcd72b45dafa 100644 (file)
--- a/gcc/config/riscv/riscv.md
+++ b/gcc/config/riscv/riscv.md
@@ -2286,12 +2286,16 @@
       rtx abs_reg = gen_reg_rtx (<ANYF:MODE>mode);
       rtx coeff_reg = gen_reg_rtx (<ANYF:MODE>mode);
       rtx tmp_reg = gen_reg_rtx (<ANYF:MODE>mode);
+      rtx fflags = gen_reg_rtx (SImode);
 
       riscv_emit_move (tmp_reg, operands[1]);
       riscv_emit_move (coeff_reg,
                       riscv_vector::get_fp_rounding_coefficient (<ANYF:MODE>mode));
       emit_insn (gen_abs<ANYF:mode>2 (abs_reg, operands[1]));
 
+      /* fp compare can set invalid flag for NaN, so backup fflags.  */
+      if (flag_trapping_math)
+        emit_insn (gen_riscv_frflags (fflags));
       riscv_expand_conditional_branch (label, LT, abs_reg, coeff_reg);
 
       emit_jump_insn (gen_jump (end_label));
@@ -2317,6 +2321,14 @@
       emit_insn (gen_copysign<ANYF:mode>3 (tmp_reg, abs_reg, operands[1]));
 
       emit_label (end_label);
+
+      /* Restore fflags, but after label.  This is slightly different
+         than glibc implementation which only needs to restore under
+         the label, since it checks for NaN first, meaning following fp
+         compare can't raise fp exceptons and thus not clobber fflags.  */
+      if (flag_trapping_math)
+        emit_insn (gen_riscv_fsflags (fflags));
+
       riscv_emit_move (operands[0], tmp_reg);
     }
 

diff --git a/gcc/testsuite/gcc.target/riscv/rvv/autovec/vls/math-nearbyint-1.c b/gcc/testsuite/gcc.target/riscv/rvv/autovec/vls/math-nearbyint-1.c
index bb62ce2ef8a8f1cbe7b31f469cea0ce20f246f36..89af160112c99240ec4e76d945a20f97a4d4a82e 100644 (file)
--- a/gcc/testsuite/gcc.target/riscv/rvv/autovec/vls/math-nearbyint-1.c
+++ b/gcc/testsuite/gcc.target/riscv/rvv/autovec/vls/math-nearbyint-1.c
@@ -54,5 +54,5 @@ DEF_OP_V (nearbyint, 512, double, __builtin_nearbyint)
 /* { dg-final { scan-tree-dump-not "4096,4096" "optimized" } } */
 /* { dg-final { scan-assembler-times {vfcvt\.x\.f\.v\s+v[0-9]+,\s*v[0-9]+,\s*v0\.t} 30 } } */
 /* { dg-final { scan-assembler-times {vfcvt\.f\.x\.v\s+v[0-9]+,\s*v[0-9]+,\s*v0\.t} 30 } } */
-/* { dg-final { scan-assembler-times {frflags\s+[atx][0-9]+} 30 } } */
-/* { dg-final { scan-assembler-times {fsflags\s+[atx][0-9]+} 30 } } */
+/* { dg-final { scan-assembler-times {frflags\s+[atx][0-9]+} 32 } } */
+/* { dg-final { scan-assembler-times {fsflags\s+[atx][0-9]+} 32 } } */