On Tue, 11 Mar 2025, Jakub Jelinek wrote:
> Hi!
>
> The PR119190 patch I've posted regresses the PR119120 testcase (not adding
> to testsuite, as it is fairly hard to scan for that problem).
> The issue is that for the partial setting of _Complex floating vars
> through __real__ on it first and __imag__ later (or vice versa) and since
> we forced all complex vars into SSA form we often have undefined (D)
> arguments of those COMPLEX_EXPRs. When we don't DCE them (for -O0 debug
> info reasons), their expansion will copy both the real and imag parts
> using the floating mode and on some targets like 387 that copying alone can
> unfortunately trigger exceptions on sNaNs or other problematic bit patterns
> and for uninitialized memory it can be triggered randomly based on whatever
> is on the stack before.
>
> The following patch deals just with the unused COMPLEX_EXPRs at -O0,
> if it is used, either complex lowering should have replaced its parts or
> it represents a user store somewhere, or passing to function argument etc.,
> all that should be really avoided for partially uninitialized complex.
> And the patch attempts to find the uninitialized arguments (unfortunately
> at -O0 we don't forward propagate the (D) SSA_NAMEs) and only copy the
> initialized parts. Another option would be to arrange for the copying
> to be done in integral mode rather than floating, kind of VCE and back.
> Though, not sure how to handle then XFmode or on 32-bit targets TFmode
> when there is no corresponding intergral mode or it isn't usable.
>
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
> Or shall I go the VCE route instead?
I think the patch as-is is more robust, but still - ugh ... I wonder
whether we can instead avoid introducing the COMPLEX_EXPR at all
at -O0?
> 2025-03-11 Jakub Jelinek <ja...@redhat.com>
>
> PR target/119120
> * cfgexpand.cc (expand_gimple_stmt_1): At -O0 expand a COMPLEX_EXPR
> with zero uses of lhs if it has complex floating point mode as
> stores of just the parts which aren't known to be uninitialized.
>
> --- gcc/cfgexpand.cc.jj 2025-01-07 20:11:04.632662813 +0100
> +++ gcc/cfgexpand.cc 2025-03-10 21:28:01.071078448 +0100
> @@ -4294,6 +4294,70 @@ expand_gimple_stmt_1 (gimple *stmt)
> if (GET_CODE (target) == SUBREG && SUBREG_PROMOTED_VAR_P (target))
> promoted = true;
>
> + /* At -O0 an unused COMPLEX_EXPR might be kept in the IL by
> + cplxlower0 pass to ensure correct debug info. If one or both
> + arguments of COMPLEX_EXPR is unitialized and it is a complex
> + floating-point mode, don't actually copy the uninitialized
> + part(s) using floating-point mode, as that could cause extra
> + exceptions. */
> + if (!optimize
> + && gimple_assign_rhs_code (assign_stmt) == COMPLEX_EXPR
> + && TREE_CODE (lhs) == SSA_NAME
> + && has_zero_uses (lhs)
> + && MEM_P (target)
> + && SCALAR_FLOAT_MODE_P (GET_MODE_INNER (GET_MODE (target))))
> + {
> + op0 = gimple_assign_rhs1 (assign_stmt);
> + tree op1 = gimple_assign_rhs2 (assign_stmt);
> + gimple *g;
> + bool ignore_op0 = (TREE_CODE (op0) == SSA_NAME
> + && SSA_NAME_IS_DEFAULT_DEF (op0)
> + && (!SSA_NAME_VAR (op0)
> + || VAR_P (SSA_NAME_VAR (op0))));
> + bool ignore_op1 = (TREE_CODE (op1) == SSA_NAME
> + && SSA_NAME_IS_DEFAULT_DEF (op1)
> + && (!SSA_NAME_VAR (op1)
> + || VAR_P (SSA_NAME_VAR (op1))));
> + tree tem = op0;
> + while (!ignore_op0
> + && TREE_CODE (tem) == SSA_NAME
> + && (g = SSA_NAME_DEF_STMT (tem))
> + && is_gimple_assign (g)
> + && gimple_assign_rhs_code (g) == SSA_NAME)
> + {
> + tem = gimple_assign_rhs1 (g);
> + if (SSA_NAME_IS_DEFAULT_DEF (tem)
> + && (!SSA_NAME_VAR (tem) || VAR_P (SSA_NAME_VAR (tem))))
> + ignore_op0 = true;
> + }
> + tem = op1;
> + while (!ignore_op1
> + && TREE_CODE (tem) == SSA_NAME
> + && (g = SSA_NAME_DEF_STMT (tem))
> + && is_gimple_assign (g)
> + && gimple_assign_rhs_code (g) == SSA_NAME)
> + {
> + tem = gimple_assign_rhs1 (g);
> + if (SSA_NAME_IS_DEFAULT_DEF (tem)
> + && (!SSA_NAME_VAR (tem) || VAR_P (SSA_NAME_VAR (tem))))
> + ignore_op1 = true;
> + }
> + if (ignore_op0 || ignore_op1)
> + {
> + if (!ignore_op0)
> + {
> + rtx rop0 = expand_normal (op0);
> + write_complex_part (target, rop0, 0, true);
> + }
> + else if (!ignore_op1)
> + {
> + rtx rop1 = expand_normal (op1);
> + write_complex_part (target, rop1, 1, true);
> + }
> + break;
> + }
> + }
> +
> /* If we store into a promoted register, don't directly
> expand to target. */
> temp = promoted ? NULL_RTX : target;
>
> Jakub
>
>
--
Richard Biener <rguent...@suse.de>
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