GCC has a generic cmpmemsi expansion via the by-pieces framework,
which shows some room for target-specific optimizations.
E.g. for comparing two aligned memory blocks of 15 bytes
we get the following sequence:
my_mem_cmp_aligned_15:
li a4,0
j .L2
.L8:
bgeu a4,a7,.L7
.L2:
add a2,a0,a4
add a3,a1,a4
lbu a5,0(a2)
lbu a6,0(a3)
addi a4,a4,1
li a7,15 // missed hoisting
subw a5,a5,a6
andi a5,a5,0xff // useless
beq a5,zero,.L8
lbu a0,0(a2) // loading again!
lbu a5,0(a3) // loading again!
subw a0,a0,a5
ret
.L7:
li a0,0
ret
Diff first byte: 15 insns
Diff second byte: 25 insns
No diff: 25 insns
Possible improvements:
* unroll the loop and use load-with-displacement to avoid offset increments
* load and compare multiple (aligned) bytes at once
* Use the bitmanip/strcmp result calculation (reverse words and
synthesize (a2 >= a3) ? 1 : -1 in a branchless sequence)
When applying these improvements we get the following sequence:
my_mem_cmp_aligned_15:
ld a5,0(a0)
ld a4,0(a1)
bne a5,a4,.L2
ld a5,8(a0)
ld a4,8(a1)
slli a5,a5,8
slli a4,a4,8
bne a5,a4,.L2
li a0,0
.L3:
sext.w a0,a0
ret
.L2:
rev8 a5,a5
rev8 a4,a4
sltu a5,a5,a4
neg a5,a5
ori a0,a5,1
j .L3
Diff first byte: 11 insns
Diff second byte: 16 insns
No diff: 11 insns
This patch implements this improvements.
The tests consist of a execution test (similar to
gcc/testsuite/gcc.dg/torture/inline-mem-cmp-1.c) and a few tests
that test the expansion conditions (known length and alignment).
Similar to the cpymemsi expansion this patch does not introduce any
gating for the cmpmemsi expansion (on top of requiring the known length,
alignment and Zbb).
Bootstrapped and SPEC CPU 2017 tested.
gcc/ChangeLog:
* config/riscv/riscv-protos.h (riscv_expand_block_compare): New
prototype.
* config/riscv/riscv-string.cc (GEN_EMIT_HELPER2): New helper.
(do_load_from_addr): Add support for HI and SI/64 modes.
(emit_memcmp_scalar_load_and_compare): New helper to emit memcmp.
(emit_memcmp_scalar_result_calculation): Likewise.
(riscv_expand_block_compare_scalar): Likewise.
(riscv_expand_block_compare): New RISC-V expander for memory compare.
* config/riscv/riscv.md (cmpmemsi): New cmpmem expansion.
gcc/testsuite/ChangeLog:
* gcc.target/riscv/cmpmemsi-1.c: New test.
* gcc.target/riscv/cmpmemsi-2.c: New test.
* gcc.target/riscv/cmpmemsi-3.c: New test.
* gcc.target/riscv/cmpmemsi.c: New test.
Signed-off-by: Christoph Müllner <christoph.muell...@vrull.eu>
---
gcc/config/riscv/riscv-protos.h | 1 +
gcc/config/riscv/riscv-string.cc | 161 ++++++++++++++++++++
gcc/config/riscv/riscv.md | 15 ++
gcc/testsuite/gcc.target/riscv/cmpmemsi-1.c | 6 +
gcc/testsuite/gcc.target/riscv/cmpmemsi-2.c | 42 +++++
gcc/testsuite/gcc.target/riscv/cmpmemsi-3.c | 43 ++++++
gcc/testsuite/gcc.target/riscv/cmpmemsi.c | 22 +++
7 files changed, 290 insertions(+)
create mode 100644 gcc/testsuite/gcc.target/riscv/cmpmemsi-1.c
create mode 100644 gcc/testsuite/gcc.target/riscv/cmpmemsi-2.c
create mode 100644 gcc/testsuite/gcc.target/riscv/cmpmemsi-3.c
create mode 100644 gcc/testsuite/gcc.target/riscv/cmpmemsi.c
diff --git a/gcc/config/riscv/riscv-protos.h b/gcc/config/riscv/riscv-protos.h
index e5aebf3fc3d..30ffe30be1d 100644
--- a/gcc/config/riscv/riscv-protos.h
+++ b/gcc/config/riscv/riscv-protos.h
@@ -188,6 +188,7 @@ rtl_opt_pass * make_pass_avlprop (gcc::context *ctxt);
rtl_opt_pass * make_pass_vsetvl (gcc::context *ctxt);
/* Routines implemented in riscv-string.c. */
+extern bool riscv_expand_block_compare (rtx, rtx, rtx, rtx);
extern bool riscv_expand_block_move (rtx, rtx, rtx);
/* Information about one CPU we know about. */
diff --git a/gcc/config/riscv/riscv-string.cc b/gcc/config/riscv/riscv-string.cc
index b09b51d7526..9d4dc0cb827 100644
--- a/gcc/config/riscv/riscv-string.cc
+++ b/gcc/config/riscv/riscv-string.cc
@@ -86,6 +86,7 @@ GEN_EMIT_HELPER2(th_rev) /* do_th_rev2 */
GEN_EMIT_HELPER2(th_tstnbz) /* do_th_tstnbz2 */
GEN_EMIT_HELPER3(xor) /* do_xor3 */
GEN_EMIT_HELPER2(zero_extendqi) /* do_zero_extendqi2 */
+GEN_EMIT_HELPER2(zero_extendhi) /* do_zero_extendhi2 */
#undef GEN_EMIT_HELPER2
#undef GEN_EMIT_HELPER3
@@ -109,6 +110,10 @@ do_load_from_addr (machine_mode mode, rtx dest, rtx
addr_reg, rtx addr)
if (mode == QImode)
do_zero_extendqi2 (dest, mem);
+ else if (mode == HImode)
+ do_zero_extendhi2 (dest, mem);
+ else if (mode == SImode && TARGET_64BIT)
+ emit_insn (gen_zero_extendsidi2 (dest, mem));
else if (mode == Xmode)
emit_move_insn (dest, mem);
else
@@ -610,6 +615,162 @@ riscv_expand_strlen (rtx result, rtx src, rtx
search_char, rtx align)
return false;
}
+static void
+emit_memcmp_scalar_load_and_compare (rtx result, rtx src1, rtx src2,
+ unsigned HOST_WIDE_INT nbytes,
+ rtx data1, rtx data2,
+ rtx diff_label, rtx final_label)
+{
+ const unsigned HOST_WIDE_INT xlen = GET_MODE_SIZE (Xmode);
+ rtx src1_addr = force_reg (Pmode, XEXP (src1, 0));
+ rtx src2_addr = force_reg (Pmode, XEXP (src2, 0));
+ unsigned HOST_WIDE_INT offset = 0;
+
+ while (nbytes > 0)
+ {
+ unsigned HOST_WIDE_INT cmp_bytes = xlen < nbytes ? xlen : nbytes;
+ machine_mode load_mode;
+
+ /* Special cases to avoid masking of trailing bytes. */
+ if (cmp_bytes == 1)
+ load_mode = QImode;
+ else if (cmp_bytes == 2)
+ load_mode = HImode;
+ else if (cmp_bytes == 4)
+ load_mode = SImode;
+ else
+ load_mode = Xmode;
+
+ rtx addr1 = gen_rtx_PLUS (Pmode, src1_addr, GEN_INT (offset));
+ do_load_from_addr (load_mode, data1, addr1, src1);
+ rtx addr2 = gen_rtx_PLUS (Pmode, src2_addr, GEN_INT (offset));
+ do_load_from_addr (load_mode, data2, addr2, src2);
