From fdd98d88d4559b80b127502e15aa1bb7ca0bf9ff Mon Sep 17 00:00:00 2001
From: Paul Mackerras <paulus@ozlabs.org>
Date: Wed, 14 Jan 2026 08:57:07 +1100
Subject: [PATCH] FPU: Fix zero result detection in fmadd-family instructions

With the multiply-add instructions, it is possible to get into state
FMADD_6 with R containing a value >= 8.0.  If the value is exactly
8.0, the logic will incorrectly conclude that the result is zero
because it only tests bits up to UNIT_BIT + 2.  Fix this by testing
up to UNIT_BIT + 3, and add a test case to the FPU test that triggers
this situation.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
---
 fpu.vhdl           |   2 +-
 tests/fpu/fpu.c    |   2 ++
 tests/test_fpu.bin | Bin 34680 -> 34744 bytes
 3 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/fpu.vhdl b/fpu.vhdl
index 190f4a3..d8b95f2 100644
--- a/fpu.vhdl
+++ b/fpu.vhdl
@@ -2015,7 +2015,7 @@ begin
                 set_r := '0';
                 opsel_r <= RES_SHIFT;
                 re_sel2 <= REXP2_NE;
-                if (r.r(UNIT_BIT + 2) or r_hi_nz or r_lo_nz or (or (r.r(DP_LSB - 1 downto 0)))) = '0' then
+                if (or (r.r(UNIT_BIT + 3 downto 0))) = '0' then
                     -- R is all zeroes but there may be non-zero bits in S
                     -- so shift them into R and set S to 0
                     set_r := '1';
diff --git a/tests/fpu/fpu.c b/tests/fpu/fpu.c
index a123f62..cdbc7a1 100644
--- a/tests/fpu/fpu.c
+++ b/tests/fpu/fpu.c
@@ -1629,6 +1629,8 @@ struct fmavals {
 	  0x41fffffffff00000, 0xc3ffffffffe00000, 0xc1fffffffff00000, 0x43ffffffffe00000 },
 	{ 0x3ff0000000000000, 0x000060fbffffefc1, 0x000060fbffffefc1, FPS_RN_NEAR,
 	  0x0000c1f7ffffdf82, 0x0000000000000000, 0x8000c1f7ffffdf82, 0x8000000000000000 },
+	{ 0x80000000fffffffd, 0x3ff0000000000000, 0x8000000300000003, FPS_UE | FPS_RN_NEAR,
+	  0xdef0000000000000, 0x5ee0000000300000, 0x5ef0000000000000, 0xdee0000000300000 },
 };
 
 int test23(long arg)
diff --git a/tests/test_fpu.bin b/tests/test_fpu.bin
index cd1d6476befa3ae556c49f6fea75b975b3fb7b09..d48077e277f22a941005c42462339a84971c02d3 100755
GIT binary patch
delta 775
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z!D2^6r4)Y-hV=)_2$_o_9t17edg>usifGBAXpTxL>)TDWQ5<;l-uHcP=6yTtkIPhy
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zN!Ywu{%To=ZET0#D~TlXud(+{d9F??u4<1{w+jG9`RRYOZz&zMOXVADn#URB7VK?c
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z($E0Ao9yz}JbOE9U&KEquWwUVEXzWyW8kXpcj1nq%QpP8stW)yM2iD8s*2gc3+x@)
J%*O}JwqGos_=Ere

delta 710
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zMi-iTMe2j1T}T;JC|4LaN^v38f^k<N3PH4VP;{#l8P7~AjpAbNJ?H=ax#yqDJiW$#
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