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* [PATCH v2] RISC-V: bitmanip: improve constant-loading for (1ULL << 31) in DImode
@ 2022-05-29 21:51 Philipp Tomsich
  2022-06-02 13:17 ` Kito Cheng
  0 siblings, 1 reply; 6+ messages in thread
From: Philipp Tomsich @ 2022-05-29 21:51 UTC (permalink / raw)
  To: gcc-patches
  Cc: Andrew Pinski, Kito Cheng, Palmer Dabbelt, Vineet Gupta, Philipp Tomsich

The SINGLE_BIT_MASK_OPERAND() is overly restrictive, triggering for
bits above 31 only (to side-step any issues with the negative SImode
value 0x80000000/(-1ull << 31)/(1 << 31)).  This moves the special
handling of this SImode value (i.e. the check for (-1ull << 31) to
riscv.cc and relaxes the SINGLE_BIT_MASK_OPERAND() test.

With this, the code-generation for loading (1ULL << 31) from:
	li	a0,1
	slli	a0,a0,31
to:
	bseti	a0,zero,31

gcc/ChangeLog:

	* config/riscv/riscv.cc (riscv_build_integer_1): Rewrite value as
	(-1 << 31) for the single-bit case, when operating on (1 << 31)
	in SImode.
	* gcc/config/riscv/riscv.h (SINGLE_BIT_MASK_OPERAND): Allow for
	any single-bit value, moving the special case for (1 << 31) to
	riscv_build_integer_1 (in riscv.c).

Signed-off-by: Philipp Tomsich <philipp.tomsich@vrull.eu>

---

Changes in v2:
- Use HOST_WIDE_INT_1U/HOST_WIDE_INT_M1U instead of constants.
- Fix some typos in the comment above the rewrite of the value.
- Update the comment to clarify that we expect a LUI to be emitted for
  the SImode case (i.e. sign-extended for RV64) of (1 << 31).

 gcc/config/riscv/riscv.cc |  9 +++++++++
 gcc/config/riscv/riscv.h  | 11 ++++-------
 2 files changed, 13 insertions(+), 7 deletions(-)

diff --git a/gcc/config/riscv/riscv.cc b/gcc/config/riscv/riscv.cc
index f83dc796d88..2e83ca07394 100644
--- a/gcc/config/riscv/riscv.cc
+++ b/gcc/config/riscv/riscv.cc
@@ -420,6 +420,15 @@ riscv_build_integer_1 (struct riscv_integer_op codes[RISCV_MAX_INTEGER_OPS],
       /* Simply BSETI.  */
       codes[0].code = UNKNOWN;
       codes[0].value = value;
+
+      /* RISC-V sign-extends all 32bit values that live in a 32bit
+	 register.  To avoid paradoxes, we thus need to use the
+	 sign-extended (negative) representation (-1 << 31) for the
+	 value, if we want to build (1 << 31) in SImode.  This will
+	 then expand to an LUI instruction.  */
+      if (mode == SImode && value == (HOST_WIDE_INT_1U << 31))
+	codes[0].value = (HOST_WIDE_INT_M1U << 31);
+
       return 1;
     }
 
diff --git a/gcc/config/riscv/riscv.h b/gcc/config/riscv/riscv.h
index 5083a1c24b0..6f7f4d3fbdc 100644
--- a/gcc/config/riscv/riscv.h
+++ b/gcc/config/riscv/riscv.h
@@ -528,13 +528,10 @@ enum reg_class
   (((VALUE) | ((1UL<<31) - IMM_REACH)) == ((1UL<<31) - IMM_REACH)	\
    || ((VALUE) | ((1UL<<31) - IMM_REACH)) + IMM_REACH == 0)
 
-/* If this is a single bit mask, then we can load it with bseti.  But this
-   is not useful for any of the low 31 bits because we can use addi or lui
-   to load them.  It is wrong for loading SImode 0x80000000 on rv64 because it
-   needs to be sign-extended.  So we restrict this to the upper 32-bits
-   only.  */
-#define SINGLE_BIT_MASK_OPERAND(VALUE) \
-  (pow2p_hwi (VALUE) && (ctz_hwi (VALUE) >= 32))
+/* If this is a single bit mask, then we can load it with bseti.  Special
+   handling of SImode 0x80000000 on RV64 is done in riscv_build_integer_1. */
+#define SINGLE_BIT_MASK_OPERAND(VALUE)					\
+  (pow2p_hwi (VALUE))
 
 /* Stack layout; function entry, exit and calling.  */
 
-- 
2.34.1


^ permalink raw reply	[flat|nested] 6+ messages in thread

end of thread, other threads:[~2022-06-14 11:18 UTC | newest]

Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2022-05-29 21:51 [PATCH v2] RISC-V: bitmanip: improve constant-loading for (1ULL << 31) in DImode Philipp Tomsich
2022-06-02 13:17 ` Kito Cheng
2022-06-02 19:23   ` Philipp Tomsich
2022-06-02 19:24     ` Philipp Tomsich
2022-06-07 10:27       ` Kito Cheng
2022-06-14 11:18         ` Philipp Tomsich

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