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Re: [PATCH v3 02/21] target/riscv: Clean up division helpers


From: Richard Henderson
Subject: Re: [PATCH v3 02/21] target/riscv: Clean up division helpers
Date: Thu, 19 Aug 2021 07:23:48 -1000
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0

On 8/19/21 1:00 AM, Bin Meng wrote:
  static void gen_rem(TCGv ret, TCGv source1, TCGv source2)
  {
-    TCGv cond1, cond2, zeroreg, resultopt1;
+    TCGv temp1, temp2, zero, one, mone, min;

-    cond1 = tcg_temp_new();
-    cond2 = tcg_temp_new();
-    zeroreg = tcg_constant_tl(0);
-    resultopt1 = tcg_temp_new();
+    temp1 = tcg_temp_new();
+    temp2 = tcg_temp_new();
+    zero = tcg_constant_tl(0);
+    one = tcg_constant_tl(1);
+    mone = tcg_constant_tl(-1);
+    min = tcg_constant_tl(1ull << (TARGET_LONG_BITS - 1));

-    tcg_gen_movi_tl(resultopt1, 1L);
-    tcg_gen_setcondi_tl(TCG_COND_EQ, cond2, source2, (target_ulong)-1);
-    tcg_gen_setcondi_tl(TCG_COND_EQ, cond1, source1,
-                        (target_ulong)1 << (TARGET_LONG_BITS - 1));
-    tcg_gen_and_tl(cond2, cond1, cond2); /* cond1 = overflow */
-    tcg_gen_setcondi_tl(TCG_COND_EQ, cond1, source2, 0); /* cond2 = div 0 */
-    /* if overflow or div by zero, set source2 to 1, else don't change */
-    tcg_gen_or_tl(cond2, cond1, cond2);
-    tcg_gen_movcond_tl(TCG_COND_EQ, source2, cond2, zeroreg, source2,
-            resultopt1);
-    tcg_gen_rem_tl(resultopt1, source1, source2);
-    /* if div by zero, just return the original dividend */
-    tcg_gen_movcond_tl(TCG_COND_EQ, ret, cond1, zeroreg, resultopt1,
-            source1);
+    tcg_gen_setcond_tl(TCG_COND_EQ, temp1, source1, min);
+    tcg_gen_setcond_tl(TCG_COND_EQ, temp2, source2, mone);
+    tcg_gen_and_tl(temp1, temp1, temp2); /* temp1 = overflow */
+    tcg_gen_setcond_tl(TCG_COND_EQ, temp2, source2, zero); /* temp2 = div0 */
+    tcg_gen_or_tl(temp2, temp2, temp1);  /* temp2 = overflow | div0 */

-    tcg_temp_free(cond1);
-    tcg_temp_free(cond2);
-    tcg_temp_free(resultopt1);
+    /*
+     * if overflow or div by zero, set temp2 to 1, else source2
+     * this automatically takes care of returning the original
+     * dividend for div by zero.
+     */
+    tcg_gen_movcond_tl(TCG_COND_NE, temp2, temp2, zero, one, source2);

What about the overflow case? The return value should be 0.

Hmm. I see that in the table, yes, but I don't see that the original code got that correct either. I'll send a follow-up patch.

r~



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