[gcc r17-3495] RISC-V: Reject table-based CRC optabs for size

https://gcc.gnu.org/g:f74150ec28282824b0a9bd298b87210e880ddec3

commit r17-3495-gf74150ec28282824b0a9bd298b87210e880ddec3
Author: Jojo R <[email protected]>
Date:   Wed Aug 19 15:01:51 2026 +0800

    RISC-V: Reject table-based CRC optabs for size
    
    The profitability check added for PR middle-end/124900 asks the target
    whether a CRC optab is suitable for both speed and size.  The RISC-V
    reversed CRC optab is always present, however, and falls back to a
    256-entry lookup table when CLMUL expansion is unavailable.  This
    allows the transformation under -Os and -Oz even though it
    significantly increases code size.
    
    Implement TARGET_OPTAB_SUPPORTED_P for crc_rev_optab.  Keep the table
    fallback available when optimizing for speed, but report the optab as
    suitable for both speed and size only when Zbc, Zbkc, or Zvbc can
    provide the CLMUL expansion and the result mode is narrower than
    word_mode.
    
    Add an RV32/RV64 regression test for the no-CLMUL -Os and -Oz paths.
    
    gcc/ChangeLog:
    
            * config/riscv/riscv.cc (riscv_optab_supported_p): New function.
            (TARGET_OPTAB_SUPPORTED_P): Define.
    
    gcc/testsuite/ChangeLog:
    
            * gcc.target/riscv/crc-size-no-table.c: New test.

Diff:
---
 gcc/config/riscv/riscv.cc                          | 18 +++++++++++++
 gcc/testsuite/gcc.target/riscv/crc-size-no-table.c | 31 ++++++++++++++++++++++
 2 files changed, 49 insertions(+)

diff --git a/gcc/config/riscv/riscv.cc b/gcc/config/riscv/riscv.cc
index d61461e256e2..ed372da98192 100644
--- a/gcc/config/riscv/riscv.cc
+++ b/gcc/config/riscv/riscv.cc
@@ -13197,6 +13197,21 @@ riscv_scalar_mode_supported_p (scalar_mode mode)
     return default_scalar_mode_supported_p (mode);
 }
 
+/* Implement TARGET_OPTAB_SUPPORTED_P.  */
+
+static bool
+riscv_optab_supported_p (int op, machine_mode, machine_mode result_mode,
+			 optimization_type opt_type)
+{
+  /* The second CRC optab mode is the result mode.  The CLMUL expansion
+     requires room for a quotient wider than the CRC value itself.  */
+  if (op == crc_rev_optab && opt_type != OPTIMIZE_FOR_SPEED)
+    return ((TARGET_ZBKC || TARGET_ZBC || TARGET_ZVBC)
+	    && result_mode < word_mode);
+
+  return true;
+}
+
 /* Implement TARGET_LIBGCC_FLOATING_MODE_SUPPORTED_P - return TRUE
    if MODE is HFmode or BFmode, and punt to the generic implementation
    otherwise.  */
@@ -16859,6 +16874,9 @@ riscv_memtag_tag_bitsize ()
 #undef TARGET_SCALAR_MODE_SUPPORTED_P
 #define TARGET_SCALAR_MODE_SUPPORTED_P riscv_scalar_mode_supported_p
 
+#undef TARGET_OPTAB_SUPPORTED_P
+#define TARGET_OPTAB_SUPPORTED_P riscv_optab_supported_p
+
 #undef TARGET_LIBGCC_FLOATING_MODE_SUPPORTED_P
 #define TARGET_LIBGCC_FLOATING_MODE_SUPPORTED_P                                \
   riscv_libgcc_floating_mode_supported_p
diff --git a/gcc/testsuite/gcc.target/riscv/crc-size-no-table.c b/gcc/testsuite/gcc.target/riscv/crc-size-no-table.c
new file mode 100644
index 000000000000..1e6d8c455293
--- /dev/null
+++ b/gcc/testsuite/gcc.target/riscv/crc-size-no-table.c
@@ -0,0 +1,31 @@
+/* { dg-do compile } */
+/* { dg-options "-march=rv32gc -mabi=ilp32 -fdump-tree-crc-details -fdisable-tree-phiopt2 -fdisable-tree-phiopt3" { target { rv32 } } } */
+/* { dg-options "-march=rv64gc -mabi=lp64 -fdump-tree-crc-details -fdisable-tree-phiopt2 -fdisable-tree-phiopt3" { target { rv64 } } } */
+/* { dg-skip-if "" { *-*-* } { "*" } { "-Os" "-Oz" } } */
+
+typedef unsigned char uint8_t;
+typedef unsigned int uint32_t;
+
+uint32_t
+crc32r (const uint8_t *data, uint32_t size)
+{
+  uint32_t crc = 0xffffffff;
+
+  for (uint32_t i = 0; i != size; i++)
+    {
+      crc ^= data[i];
+      for (int j = 0; j < 8; j++)
+	if (crc & 1)
+	  crc = (crc >> 1) ^ 0xedb88320;
+	else
+	  crc >>= 1;
+    }
+
+  return ~crc;
+}
+
+/* The CRC loop must be recognized, but retained because rv32gc/rv64gc has no
+   CRC optab that is smaller than the original loop.  */
+/* { dg-final { scan-tree-dump "calculates CRC!" "crc" } } */
+/* { dg-final { scan-tree-dump "Couldn't generate faster CRC code." "crc" } } */
+/* { dg-final { scan-tree-dump-not {\.CRC_REV} "crc" } } */