Attached is what I have for carry_backpropagate .
The utility of special handling for SS_ASHIFT / US_ASHIFT seems
somewhat marginal.
I suspect it'd be more useful to add handling of LSHIFTRT and ASHIFTRT
. Some ports do
a lot of static shifting.
commit ed47c3d0d38f85c9b4e22bdbd079e0665465ef9c
Author: Joern Rennecke <joern.renne...@embecosm.com>
Date: Wed Nov 29 18:46:06 2023 +0000
* ext-dce.c: Fixes for carry handling.
* ext-dce.c (safe_for_live_propagation): Handle MINUS.
(ext_dce_process_uses): Break out carry handling into ..
(carry_backpropagate): This new function.
Better handling of ASHIFT.
Add handling of SMUL_HIGHPART, UMUL_HIGHPART, SIGN_EXTEND, SS_ASHIFT and
US_ASHIFT.
diff --git a/gcc/ext-dce.cc b/gcc/ext-dce.cc
index 590656f72c7..2a4508181a1 100644
--- a/gcc/ext-dce.cc
+++ b/gcc/ext-dce.cc
@@ -83,6 +83,7 @@ safe_for_live_propagation (rtx_code code)
case SIGN_EXTEND:
case TRUNCATE:
case PLUS:
+ case MINUS:
case MULT:
case SMUL_HIGHPART:
case UMUL_HIGHPART:
@@ -365,6 +366,67 @@ binop_implies_op2_fully_live (rtx_code code)
}
}
+/* X, with code CODE, is an operation for which
+safe_for_live_propagation holds true,
+ and bits set in MASK are live in the result. Compute a make of
(potentially)
+ live bits in the non-constant inputs. In case of
+binop_implies_op2_fully_live
+ (e.g. shifts), the computed mask may exclusively pertain to the
+first operand. */
+
+HOST_WIDE_INT
+carry_backpropagate (HOST_WIDE_INT mask, enum rtx_code code, rtx x)
+{
+ enum machine_mode mode = GET_MODE (x);
+ HOST_WIDE_INT mmask = GET_MODE_MASK (mode);
+ switch (code)
+ {
+ case ASHIFT:
+ if (CONSTANT_P (XEXP (x, 1))
+ && known_lt (UINTVAL (XEXP (x, 1)), GET_MODE_BITSIZE (mode)))
+ return mask >> INTVAL (XEXP (x, 1));
+ /* Fall through. */
+ case PLUS: case MINUS:
+ case MULT:
+ return mask ? ((2ULL << floor_log2 (mask)) - 1) : 0;
+ case SMUL_HIGHPART: case UMUL_HIGHPART:
+ if (!mask || XEXP (x, 1) == const0_rtx)
+ return 0;
+ if (CONSTANT_P (XEXP (x, 1)))
+ {
+ if (pow2p_hwi (INTVAL (XEXP (x, 1))))
+ return mmask & (mask << (GET_MODE_BITSIZE (mode).to_constant ()
+ - exact_log2 (INTVAL (XEXP (x, 1)))));
+
+ int bits = (2 * GET_MODE_BITSIZE (mode).to_constant ()
+ - clz_hwi (mask) - ctz_hwi (INTVAL (XEXP (x, 1))));
+ if (bits < GET_MODE_BITSIZE (mode).to_constant ())
+ return (1ULL << bits) - 1;
+ }
+ return mmask;
+ case SIGN_EXTEND:
+ if (mask & ~mmask)
+ mask |= 1ULL << (GET_MODE_BITSIZE (mode).to_constant () - 1);
+ return mask;
+
+ /* We propagate for the shifted operand, but not the shift
+ count. The count is handled specially. */
+ case SS_ASHIFT:
+ case US_ASHIFT:
+ if (!mask || XEXP (x, 1) == const0_rtx)
+ return 0;
+ if (CONSTANT_P (XEXP (x, 1))
+ && UINTVAL (XEXP (x, 1)) < GET_MODE_BITSIZE (mode).to_constant ())
+ {
+ return ((mmask & ~((unsigned HOST_WIDE_INT)mmask
+ >> (INTVAL (XEXP (x, 1)) + (code == SS_ASHIFT))))
+ | (mask >> INTVAL (XEXP (x, 1))));
+ }
+ return mmask;
+ default:
+ return mask;
+ }
+}
/* Process uses in INSN contained in OBJ. Set appropriate bits in LIVENOW
for any chunks of pseudos that become live, potentially filtering using
bits from LIVE_TMP.
@@ -480,11 +542,7 @@ ext_dce_process_uses (rtx_insn *insn, rtx obj, bitmap
livenow,
sure everything that should get marked as live is marked
from here onward. */
- /* ?!? What is the point of this adjustment to DST_MASK? */
- if (code == PLUS || code == MINUS
- || code == MULT || code == ASHIFT)
- dst_mask
- = dst_mask ? ((2ULL << floor_log2 (dst_mask)) - 1) : 0;
+ dst_mask = carry_backpropagate (dst_mask, code, src);
/* We will handle the other operand of a binary operator
at the bottom of the loop by resetting Y. */
