RISC-V: bitmanip: improve constant-loading for (1ULL << 31) in DImode

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.
	* 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>

(cherry picked from commit 4e72ccad80d69a76d149fba59603b8173fffe8fe)

2 files changed, 13 insertions, 7 deletions

diff --git a/gcc/config/riscv/riscv.cc b/gcc/config/riscv/riscv.cc
index f3ac0d8865f..4939d9964db 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 b191606edb4..b3eb6abc2aa 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.  */