soft-fp64/fadd: Rename aFrac and bFrac variables

Exchanging aFracHi / bFracHi and aFracLo / bFracLo should not affect the
result of the later call to __add64.

The main purpose of this commit is to prepare for "soft-fp64/fadd: Move
common code out of both branches of an if-statement".

v2: Fix a typo in a comment.  Noticed by Matt.

Results on the 308 shaders extracted from the fp64 portion of the OpenGL
CTS:

Tiger Lake and Ice Lake had similar results. (Tiger Lake shown)
total instructions in shared programs: 812094 -> 818246 (0.76%)
instructions in affected programs: 750271 -> 756423 (0.82%)
helped: 0
HURT: 74
HURT stats (abs)   min: 7 max: 520 x̄: 83.14 x̃: 59
HURT stats (rel)   min: 0.52% max: 1.48% x̄: 0.89% x̃: 0.84%
95% mean confidence interval for instructions value: 63.96 102.31
95% mean confidence interval for instructions %-change: 0.83% 0.95%
Instructions are HURT.

total cycles in shared programs: 6797157 -> 6816686 (0.29%)
cycles in affected programs: 6365175 -> 6384704 (0.31%)
helped: 0
HURT: 74
HURT stats (abs)   min: 16 max: 1690 x̄: 263.91 x̃: 181
HURT stats (rel)   min: 0.14% max: 0.68% x̄: 0.32% x̃: 0.27%
95% mean confidence interval for cycles value: 199.74 328.07
95% mean confidence interval for cycles %-change: 0.29% 0.36%
Cycles are HURT.

total spills in shared programs: 703 -> 705 (0.28%)
spills in affected programs: 703 -> 705 (0.28%)
helped: 0
HURT: 1

total fills in shared programs: 1499 -> 1501 (0.13%)
fills in affected programs: 1499 -> 1501 (0.13%)
helped: 0
HURT: 1

Reviewed-by: Matt Turner <[email protected]>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4142>

1 files changed, 21 insertions, 6 deletions

diff --git a/src/compiler/glsl/float64.glsl b/src/compiler/glsl/float64.glsl
index 40cc84439fb..cdecfdb8dbb 100644
--- a/src/compiler/glsl/float64.glsl
+++ b/src/compiler/glsl/float64.glsl
@@ -661,6 +661,17 @@ __propagateFloat64NaN(uint64_t __a, uint64_t __b)
 #endif
 }
 
+/* If a shader is in the soft-fp64 path, it almost certainly has register
+ * pressure problems.  Choose a method to exchange two values that does not
+ * require a temporary.
+ */
+#define EXCHANGE(a, b) \
+   do {                \
+       a ^= b;         \
+       b ^= a;         \
+       a ^= b;         \
+   } while (false)
+
 /* Returns the result of adding the double-precision floating-point values
  * `a' and `b'.  The operation is performed according to the IEEE Standard for
  * Floating-Point Arithmetic.
@@ -709,15 +720,19 @@ __fadd64(uint64_t a, uint64_t b)
                bFracHi, bFracLo, 0u, expDiff, bFracHi, bFracLo, zFrac2);
             zExp = aExp;
          } else {
-            if (bExp == 0x7FF) {
-               bool propagate = (bFracHi | bFracLo) != 0u;
+            EXCHANGE(aFracHi, bFracHi);
+            EXCHANGE(aFracLo, bFracLo);
+            EXCHANGE(aExp, bExp);
+
+            if (aExp == 0x7FF) {
+               bool propagate = (aFracHi | aFracLo) != 0u;
                return mix(__packFloat64(aSign, 0x7ff, 0u, 0u), __propagateFloat64NaN(a, b), propagate);
             }
-            expDiff = mix(expDiff, expDiff + 1, aExp == 0);
-            aFracHi = mix(aFracHi | 0x00100000u, aFracHi, aExp == 0);
+            expDiff = mix(expDiff, expDiff + 1, bExp == 0);
+            bFracHi = mix(bFracHi | 0x00100000u, bFracHi, bExp == 0);
             __shift64ExtraRightJamming(
-               aFracHi, aFracLo, 0u, - expDiff, aFracHi, aFracLo, zFrac2);
-            zExp = bExp;
+               bFracHi, bFracLo, 0u, - expDiff, bFracHi, bFracLo, zFrac2);
+            zExp = aExp;
          }
 
          aFracHi |= 0x00100000u;