https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102062
Kewen Lin <linkw at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |linkw at gcc dot gnu.org
--- Comment #8 from Kewen Lin <linkw at gcc dot gnu.org> ---
Haochen's patch r12-1202 helps to make the variable expansion work for this
case.
With GCC11, we get the RTL like:
11: NOTE_INSN_BASIC_BLOCK 3
12: r118:DI=r118:DI+0x4
13: r128:SI=[r118:DI]
14: r127:SI=r128:SI+r123:DI#0 // A
REG_DEAD r128:SI
REG_DEAD r123:DI
15: r123:DI=sign_extend(r127:SI) // B
REG_DEAD r127:SI
16: r129:SI=r119:DI#0-0x1
REG_DEAD r119:DI
17: r119:DI=zero_extend(r129:SI)
REG_DEAD r129:SI
19: r130:CC=cmp(r119:DI,0)
20: pc={(r130:CC!=0)?L35:pc}
While with trunk, we get the RTL like:
10: NOTE_INSN_BASIC_BLOCK 3
11: r118:DI=r118:DI+0x4
12: r127:SI=[r118:DI]
13: r120:SI=r120:SI+r127:SI // C
REG_DEAD r127:SI
14: r119:SI=r119:SI-0x1
16: r128:CC=cmp(r119:SI,0)
17: pc={(r128:CC!=0)?L33:pc}
We have A+B for the accumulation with GCC11 while just C with trunk. The C
pattern matches the check in function analyze_insn_to_expand_var, which is able
to record var_to_expand further.
The related code in analyze_insn_to_expand_var is:
/* Find the accumulator use within the operation. */
if (code == FMA)
{
/* We only support accumulation via FMA in the ADD position. */
if (!rtx_equal_p (dest, XEXP (src, 2)))
return NULL;
accum_pos = 2;
}
else if (rtx_equal_p (dest, XEXP (src, 0)))
accum_pos = 0;
else if (rtx_equal_p (dest, XEXP (src, 1)))
{
/* The method of expansion that we are using; which includes the
initialization of the expansions with zero and the summation of
the expansions at the end of the computation will yield wrong
results for (x = something - x) thus avoid using it in that case. */
if (code == MINUS)
return NULL;
accum_pos = 1;
}
else
return NULL;
The key is if dest can match XEXP (src, 0) or XEXP (src, 1).
