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Re: [PATCH v3 14/21] target/riscv: support for 128-bit arithmetic instru

From: Richard Henderson
Subject: Re: [PATCH v3 14/21] target/riscv: support for 128-bit arithmetic instructions
Date: Wed, 20 Oct 2021 13:15:45 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.13.0

On 10/19/21 2:48 AM, Frédéric Pétrot wrote:
+static bool gen_setcond_i128(TCGv rl, TCGv rh,
+                             TCGv al, TCGv ah,
+                             TCGv bl, TCGv bh,
+                             TCGCond cond)
+    switch (cond) {
+    case TCG_COND_EQ:
+        tcg_gen_setcond_tl(TCG_COND_EQ, rl, al, bl);
+        tcg_gen_setcond_tl(TCG_COND_EQ, rh, ah, bh);
+        tcg_gen_and_tl(rl, rl, rh);
+        break;
+    case TCG_COND_NE:
+        tcg_gen_setcond_tl(TCG_COND_NE, rl, al, bl);
+        tcg_gen_setcond_tl(TCG_COND_NE, rh, ah, bh);
+        tcg_gen_or_tl(rl, rl, rh);
+        break;

But of course setcond is more expensive than logic
Better as xor + xor + or + setcond.

+    case TCG_COND_LTU:
+    {
+        TCGv tmp1 = tcg_temp_new(),
+             tmp2 = tcg_temp_new();
+        tcg_gen_sub2_tl(rl, rh, al, ah, bl, bh);
+        tcg_gen_eqv_tl(tmp1, ah, bh);
+        tcg_gen_and_tl(tmp1, tmp1, rh);
+        tcg_gen_andc_tl(tmp2, bh, ah);
+        tcg_gen_or_tl(tmp1, tmp1, tmp2);
+        tcg_gen_shri_tl(rl, tmp1, 63);

Hmm.  Seems like it would work.
I make that 7 operations.

Or GEU in 6 operations:

    /* borrow in to second word */
    setcond_tl(TCG_COND_LTU, t1, al, bl);
    /* seed third word with 1, which will be result */
    sub2_tl(t1, t2, ah, one, t1, zero);
    sub2_tl(t1, rl, t1, t2, bh, zero);

+        } else {
+            gen_setcond_i128(tmpl, tmph, src1, src1h, src2, src2h, cond);
+            tcg_gen_brcondi_tl(TCG_COND_NE, tmpl, 0, l);
+        }

There are two setcond cases that invert their result; you should fold that in here and invert the branch condition. As long as you're special casing 0, you might as well special case TCG_COND_LT/GE and test the sign of the high word.


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